tidy-up: miscellaneous

- asyn-thrdd.c: scope an include.
- apply more clang-format suggestions.
- tidy-up PP guard comments.
- delete empty line from the top of headers.
- add empty line after `curl_setup.h` include where missing.
- fix indent.
- CODE_STYLE.md: add `strcpy`.
  Follow-up to 8636ad55df #20088
- lib1901.c: drop unnecessary line.
  Follow-up to 436e67f65b #20076

Closes #20070

lib/cf-h1-proxy.c

@@ -21,7 +21,6 @@
  * SPDX-License-Identifier: curl
  *
  ***************************************************************************/
-
 #include "curl_setup.h"
 
 #if !defined(CURL_DISABLE_PROXY) && !defined(CURL_DISABLE_HTTP)
@@ -788,4 +787,4 @@ CURLcode Curl_cf_h1_proxy_insert_after(struct Curl_cfilter *cf_at,
   return result;
 }
 
-#endif /* !CURL_DISABLE_PROXY && ! CURL_DISABLE_HTTP */
+#endif /* !CURL_DISABLE_PROXY && !CURL_DISABLE_HTTP */