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/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;