clang-tidy: sync argument names in prototype and definition

Discovered with clang-tidy checker
`readability-inconsistent-declaration-parameter-name`.

Also:
- do not enforce the above because of inconsistencies still present
  between public API prototypes and definitions. (Also betwen man page
  protos, and man page examples, and other parts of the code, e.g.
  `easy` vs `curl` vs `d` vs `handle`) Perhaps subject for a future
  effort:
  https://github.com/curl/curl/actions/runs/22166472728/job/64094691653
- enable and fix `readability-named-parameter` where missing.

Refs:
https://clang.llvm.org/extra/clang-tidy/checks/readability/inconsistent-declaration-parameter-name.html
https://clang.llvm.org/extra/clang-tidy/checks/readability/named-parameter.html

Closes #20624

lib/request.h

@@ -178,7 +178,7 @@ void Curl_req_hard_reset(struct SingleRequest *req, struct Curl_easy *data);
  * @param httpversion version used in request (09, 10, 11, etc.)
  * @return CURLE_OK (on blocking with *pnwritten == 0) or error.
  */
-CURLcode Curl_req_send(struct Curl_easy *data, struct dynbuf *buf,
+CURLcode Curl_req_send(struct Curl_easy *data, struct dynbuf *req,
                        unsigned char httpversion);
 
 /**