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/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;