Description:
------------
curl_multi_select returns -1 for error or non error condition.

If the underlying 'curl_multi_fdset' sets the max_fd to -1, curl_multi_select returns -1.  That isn't an error condition, but a 'please call curl_multi_perform soon' signal.  

If the underlying call to 'select' returns error (-1), then it is an error condition. Unfortunately there is no way from the PHP code to find out if select reported an error, or if the multi_fdset reported max_fd = -1.  

It would be useful if curl_multi_select at least set some value somewhere that would let us know that the select failed, preferably with info from errno.

  $result = curl_multi_select($cmh);
  if ($result === -1) {
     $err = curl_multi_select_last_error($cmh); // missing this function.
     if ($err) {
        throw new \Exception($err);
     }
  }