unix: fix stream refcounting buglet

Fix a buglet where uv_read_stop() would mark the handle as stopped even
when there are in-progress write requests.

This bug is unlikely to have affected anyone, the only case where it
has a user-visible effect is when:

  a) the handle has been stopped for reading but not writing, and
  b) it's the last active handle in the event loop's pollset

If both conditions are met, it's possible for the event loop to
terminate prematurely.
unix: turn off POLLOUT after stream connect

Clear the POLLOUT flag after we're done connecting. Not doing so isn't
really harmful but it may cause the event loop to wake up more often
than it has to.