multi: add curl_multi_wait()

/*
 * Name:     curl_multi_wait()
 *
 * Desc:     Poll on all fds within a CURLM set as well as any
 *           additional fds passed to the function.
 *
 * Returns:  CURLMcode type, general multi error code.
 */
CURL_EXTERN CURLMcode curl_multi_wait(CURLM *multi_handle,
                                      struct curl_waitfd extra_fds[],
                                      unsigned int extra_nfds,
                                      int timeout_ms);