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-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;