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/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;