build: use more const

Mostly with `char *` types.

Also:
- mime, x509asn1, tool_operate, lib3207: drop redundant casts.
- examples/smooth-gtk-thread: add missing variable declaration.
- reduce variable scopes.
- tests/server: move `data_to_hex()` to its only user: `sws`.

Closes #20489

docs/examples/10-at-a-time.c

@@ -131,7 +131,7 @@ int main(void)
       /* !checksrc! disable EQUALSNULL 1 */
       while((msg = curl_multi_info_read(multi, &msgs_left)) != NULL) {
         if(msg->msg == CURLMSG_DONE) {
-          char *url;
+          const char *url;
           CURL *curl = msg->easy_handle;
           curl_easy_getinfo(curl, CURLINFO_PRIVATE, &url);
           fprintf(stderr, "R: %d - %s <%s>\n",

docs/examples/anyauthput.c

@@ -88,15 +88,15 @@ static size_t read_cb(char *ptr, size_t size, size_t nmemb, void *stream)
   return nread;
 }
 
-int main(int argc, char **argv)
+int main(int argc, const char **argv)
 {
   CURL *curl;
   CURLcode result;
   FILE *fp;
   struct stat file_info;
 
-  char *file;
-  char *url;
+  const char *file;
+  const char *url;
 
   if(argc < 3)
     return 1;

docs/examples/block_ip.c

@@ -191,8 +191,8 @@ static int ip_match(struct ip *ip, void *netaddr)
   int bytes, tailbits;
   const unsigned char *x, *y;
 
-  x = (unsigned char *)&ip->netaddr;
-  y = (unsigned char *)netaddr;
+  x = (const unsigned char *)&ip->netaddr;
+  y = (const unsigned char *)netaddr;
 
   for(bytes = ip->maskbits / 8; bytes; --bytes) {
     if(*x++ != *y++)
@@ -214,7 +214,7 @@ static int is_ipv4_mapped_ipv6_address(int family, void *netaddr)
 {
   if(family == AF_INET6) {
     int i;
-    unsigned char *x = (unsigned char *)netaddr;
+    const unsigned char *x = (const unsigned char *)netaddr;
     for(i = 0; i < 12; ++i) {
       if(x[i])
         break;

docs/examples/chkspeed.c

@@ -66,13 +66,13 @@ static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *data)
   return (size_t)(size * nmemb);
 }
 
-int main(int argc, char *argv[])
+int main(int argc, const char *argv[])
 {
   CURL *curl;
   CURLcode result;
   int prtall = 0, prtsep = 0, prttime = 0;
   const char *url = URL_1M;
-  char *appname = argv[0];
+  const char *appname = argv[0];
 
   if(argc > 1) {
     /* parse input parameters */

docs/examples/ftp-wildcard.c

@@ -97,7 +97,7 @@ static size_t write_cb(char *buff, size_t size, size_t nmemb, void *cb_data)
   return written;
 }
 
-int main(int argc, char **argv)
+int main(int argc, const char **argv)
 {
   /* curl easy handle */
   CURL *curl;

docs/examples/getinfo.c

@@ -43,7 +43,7 @@ int main(void)
     result = curl_easy_perform(curl);
 
     if(CURLE_OK == result) {
-      char *ct;
+      const char *ct;
       /* ask for the content-type */
       result = curl_easy_getinfo(curl, CURLINFO_CONTENT_TYPE, &ct);
 

docs/examples/getredirect.c

@@ -33,8 +33,6 @@ int main(void)
 {
   CURL *curl;
   CURLcode result;
-  char *location;
-  long response_code;
 
   result = curl_global_init(CURL_GLOBAL_ALL);
   if(result)
@@ -53,12 +51,14 @@ int main(void)
       fprintf(stderr, "curl_easy_perform() failed: %s\n",
               curl_easy_strerror(result));
     else {
+      long response_code;
       result = curl_easy_getinfo(curl, CURLINFO_RESPONSE_CODE, &response_code);
       if((result == CURLE_OK) && ((response_code / 100) != 3)) {
         /* a redirect implies a 3xx response code */
         fprintf(stderr, "Not a redirect.\n");
       }
       else {
+        const char *location;
         result = curl_easy_getinfo(curl, CURLINFO_REDIRECT_URL, &location);
 
         if((result == CURLE_OK) && location) {

docs/examples/getreferrer.c

@@ -49,7 +49,7 @@ int main(void)
       fprintf(stderr, "curl_easy_perform() failed: %s\n",
               curl_easy_strerror(result));
     else {
-      char *hdr;
+      const char *hdr;
       result = curl_easy_getinfo(curl, CURLINFO_REFERER, &hdr);
       if((result == CURLE_OK) && hdr)
         printf("Referrer header: %s\n", hdr);

docs/examples/htmltidy.c

@@ -74,7 +74,7 @@ static void dumpNode(TidyDoc doc, TidyNode tnod, int indent)
   }
 }
 
-int main(int argc, char **argv)
+int main(int argc, const char **argv)
 {
   CURL *curl;
   char curl_errbuf[CURL_ERROR_SIZE];

docs/examples/htmltitle.cpp

@@ -250,7 +250,7 @@ static void parseHtml(const std::string &html,
   title = context.title;
 }
 
-int main(int argc, char *argv[])
+int main(int argc, const char *argv[])
 {
   CURL *curl = NULL;
   CURLcode result;

docs/examples/http2-download.c

@@ -187,7 +187,7 @@ static int setup(struct transfer *t, int num)
 /*
  * Download many transfers over HTTP/2, using the same connection!
  */
-int main(int argc, char **argv)
+int main(int argc, const char **argv)
 {
   CURLcode result;
   struct transfer *trans;

docs/examples/http2-pushinmemory.c

@@ -93,7 +93,7 @@ static int server_push_callback(CURL *parent,
                                 struct curl_pushheaders *headers,
                                 void *userp)
 {
-  char *headp;
+  const char *headp;
   int *transfers = (int *)userp;
   (void)parent;
   (void)num_headers;

docs/examples/http2-serverpush.c

@@ -172,7 +172,7 @@ static int server_push_callback(CURL *parent,
                                 struct curl_pushheaders *headers,
                                 void *userp)
 {
-  char *headp;
+  const char *headp;
   size_t i;
   int *transfers = (int *)userp;
   char filename[128];
@@ -214,7 +214,7 @@ static int server_push_callback(CURL *parent,
 /*
  * Download a file over HTTP/2, take care of server push.
  */
-int main(int argc, char *argv[])
+int main(int argc, const char *argv[])
 {
   CURLcode result;
   CURL *curl;

docs/examples/http2-upload.c

@@ -285,7 +285,7 @@ static int setup(struct input *t, int num, const char *upload)
 /*
  * Upload all files over HTTP/2, using the same physical connection!
  */
-int main(int argc, char **argv)
+int main(int argc, const char **argv)
 {
   CURLcode result;
   struct input *trans;

docs/examples/httpput-postfields.c

@@ -47,11 +47,11 @@ static const char olivertwist[] =
  * CURLOPT_POSTFIELDS to the URL given as an argument.
  */
 
-int main(int argc, char **argv)
+int main(int argc, const char **argv)
 {
   CURL *curl;
   CURLcode result;
-  char *url;
+  const char *url;
 
   if(argc < 2)
     return 1;

docs/examples/httpput.c

@@ -73,15 +73,15 @@ static size_t read_cb(char *ptr, size_t size, size_t nmemb, void *stream)
   return retcode;
 }
 
-int main(int argc, char **argv)
+int main(int argc, const char **argv)
 {
   CURL *curl;
   CURLcode result;
   FILE *hd_src;
   struct stat file_info;
 
-  char *file;
-  char *url;
+  const char *file;
+  const char *url;
 
   if(argc < 3)
     return 1;

docs/examples/multi-event.c

@@ -213,7 +213,7 @@ static int handle_socket(CURL *curl, curl_socket_t s, int action, void *userp,
   return 0;
 }
 
-int main(int argc, char **argv)
+int main(int argc, const char **argv)
 {
   CURLcode result;
 

docs/examples/multi-uv.c

@@ -227,7 +227,7 @@ static int cb_socket(CURL *curl, curl_socket_t s, int action,
   return 0;
 }
 
-int main(int argc, char **argv)
+int main(int argc, const char **argv)
 {
   CURLcode result;
   struct datauv uv = { 0 };

docs/examples/postit2-formadd.c

@@ -45,7 +45,7 @@
 
 #include <curl/curl.h>
 
-int main(int argc, char *argv[])
+int main(int argc, const char *argv[])
 {
   CURL *curl;
   CURLcode result;

docs/examples/postit2.c

@@ -42,7 +42,7 @@
 
 #include <curl/curl.h>
 
-int main(int argc, char *argv[])
+int main(int argc, const char *argv[])
 {
   CURL *curl;
 

docs/examples/smooth-gtk-thread.c

@@ -65,7 +65,7 @@ static size_t write_cb(void *ptr, size_t size, size_t nmemb, FILE *stream)
   return fwrite(ptr, size, nmemb, stream);
 }
 
-static void run_one(gchar *http, int j)
+static void run_one(const gchar *http, int j)
 {
   CURL *curl;
 
@@ -94,6 +94,7 @@ static void *pull_one_url(void *NaN)
   /* protect the reading and increasing of 'j' with a mutex */
   pthread_mutex_lock(&lock);
   while(j < num_urls) {
+    gchar *http;
     int i = j;
     j++;
     pthread_mutex_unlock(&lock);
@@ -165,7 +166,7 @@ static gboolean cb_delete(GtkWidget *window, gpointer data)
   return FALSE;
 }
 
-int main(int argc, char **argv)
+int main(int argc, const char **argv)
 {
   GtkWidget *top_window, *outside_frame, *inside_frame, *progress_bar;
 

docs/examples/sslbackend.c

@@ -40,7 +40,7 @@
  *  **** This example only works with libcurl 7.56.0 and later! ****
  */
 
-int main(int argc, char **argv)
+int main(int argc, const char **argv)
 {
   const char *name = argc > 1 ? argv[1] : "openssl";
   CURLsslset result;

docs/examples/synctime.c

@@ -126,7 +126,7 @@ static size_t SyncTime_CURL_WriteHeader(void *ptr, size_t size, size_t nmemb,
   if(ShowAllHeader == 1)
     fprintf(stderr, "%.*s", (int)nmemb, (char *)ptr);
 
-  if((nmemb >= 5) && !strncmp((char *)ptr, "Date:", 5)) {
+  if((nmemb >= 5) && !strncmp((const char *)ptr, "Date:", 5)) {
     if(ShowAllHeader == 0)
       fprintf(stderr, "HTTP Server. %.*s", (int)nmemb, (char *)ptr);
 
@@ -163,7 +163,7 @@ static size_t SyncTime_CURL_WriteHeader(void *ptr, size_t size, size_t nmemb,
     }
   }
 
-  if((nmemb >= 12) && !strncmp((char *)ptr, "X-Cache: HIT", 12)) {
+  if((nmemb >= 12) && !strncmp((const char *)ptr, "X-Cache: HIT", 12)) {
     fprintf(stderr, "ERROR: HTTP Server data is cached."
             " Server Date is no longer valid.\n");
     AutoSyncTime = 0;
@@ -226,7 +226,7 @@ static void showUsage(void)
   return;
 }
 
-int main(int argc, char *argv[])
+int main(int argc, const char *argv[])
 {
   CURLcode result;
   CURL *curl;

docs/examples/url2file.c

@@ -42,7 +42,7 @@ static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *stream)
   return written;
 }
 
-int main(int argc, char *argv[])
+int main(int argc, const char *argv[])
 {
   static const char *pagefilename = "page.out";