- Save one call to curlx_tvnow(), which calls gettimeofday(), in each of

  Curl_socket_ready(), Curl_poll() and Curl_select() when these are called
  with a zero timeout or a timeout value indicating a blocking call should
  be performed.

  These unnecessary calls to gettimeofday() got introduced in 7.16.2 when
  fixing 'timeout would restart when signal caught while awaiting socket
  events' on 20 March 2007.

- Move some loop breaking logic from the while clause into the loop,
  avoiding compiler warning 'assignment within conditional expression'