Fixes WordPress#283 : Fixes CPU busy loop when using request_multiple.

This may be the same as WordPress#110

This will require more testing across various libcurl versions.
I doubt that the timeout is necessary for curl_multi_select
(calls select() if it can),
but leaving in one just in case of bugs, so that it will end.
- Haven't thoroughly checked for relevant libcurl bugs.

Asynchronously wait for events with a short timeout if CURLM_CALL_MULTI_PERFORM
fails.

library/Requests/Transport/cURL.php

@@ -222,6 +222,14 @@ public function request_multiple($requests, $options) {
 			}
 			while ($status === CURLM_CALL_MULTI_PERFORM);
 
+			// Insert a select() with a maximum of a 50ms sleep here so that we don't busy loop and chew cpu
+			$select_res = curl_multi_select($multihandle, 0.05);
+			if ($select_res === -1) {
+				// We were unable to select() - Sleep for 1 millisecond.
+				// >  On failure, curl_multi_select will return -1 on a select failure (from the underlying select system call).
+				usleep(1000);
+			}
+
 			$to_process = array();
 
 			// Read the information as needed