Fix curl usage according to the source !

* The socket callback is not called when the action does not change that
  means one should reschedule a poll operation if a call to
  curl_multi_socket_action let unchanged the requested poll action, the
  developer must reschedule a poll operation.
* Unlike what the documentation of curl_multi_socket_action says,
  CURL_POLL_IN/OUT/INTOUT should be used instead of
  CURL_CSELECT_IN/OUT/ERR.