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/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;