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/asyn-ares.c

@@ -21,7 +21,6 @@
  * SPDX-License-Identifier: curl
  *
  ***************************************************************************/
-
 #include "curl_setup.h"
 
 #ifdef CURLRES_ARES
@@ -872,7 +871,7 @@ static CURLcode async_ares_set_dns_servers(struct Curl_easy *data,
     result = CURLE_BAD_FUNCTION_ARGUMENT;
     break;
   }
-#else /* too old c-ares version! */
+#else /* c-ares version too old! */
   (void)data;
   (void)(ares_result);
 #endif