multi: fix curl_multi_waitfds reporting of fd_count

- Make curl_multi_waitfds consistent with the documentation.

Issue Addressed:

 - The documentation of curl_multi_waitfds indicates that users should
   be able to call curl_multi_waitfds with a NULL ufds. However, before
   this change, the function would return CURLM_BAD_FUNCTION_ARGUMENT.
 - Additionally, the documentation suggests that users can use this
   function to determine the number of file descriptors (fds) needed.
   However, the function would stop counting fds if the supplied fds
   were exhausted.

Changes Made:

 - NULL ufds Handling: curl_multi_waitfds can now accept a NULL ufds if
   size is also zero.
 - Counting File Descriptors: If curl_multi_waitfds is passed a NULL
   ufds, or the size of ufds is insufficient, the output parameter
   fd_count will return the number of fds needed. This value may be
   higher than actually needed but never lower.

Testing:

 - Test 2405 has been updated to cover the usage scenarios described
   above.

Fixes https://github.com/curl/curl/issues/15146
Closes https://github.com/curl/curl/pull/15155

lib/select.h

@@ -140,8 +140,7 @@ void Curl_waitfds_init(struct curl_waitfds *cwfds,
                        struct curl_waitfd *static_wfds,
                        unsigned int static_count);
 
-CURLcode Curl_waitfds_add_ps(struct curl_waitfds *cwfds,
-                             struct easy_pollset *ps);
-
+unsigned int Curl_waitfds_add_ps(struct curl_waitfds *cwfds,
+                                 struct easy_pollset *ps);
 
 #endif /* HEADER_CURL_SELECT_H */