CURLOPT: drop redundant long casts

Also:
- CURLOPT_HSTS_CTRL.md: sync macro definitions with `curl/curl.h`.
  Perhaps it'd be better to delete copies like this?
- keep existing casts within the documentation to make sure it applies
  to older curl versions as well.
- CURLOPT_IPRESOLVE.md: re-add a long cast to man page, for consistency
  with the above.

Closes #17791

docs/examples/hsts-preload.c

@@ -87,7 +87,7 @@ int main(void)
     struct state st = {0};
 
     /* enable HSTS for this handle */
-    curl_easy_setopt(curl, CURLOPT_HSTS_CTRL, (long)CURLHSTS_ENABLE);
+    curl_easy_setopt(curl, CURLOPT_HSTS_CTRL, CURLHSTS_ENABLE);
 
     /* function to call at first to populate the cache before the transfer */
     curl_easy_setopt(curl, CURLOPT_HSTSREADFUNCTION, hstsread);