lib/async_req: make sure we return errors early from async_connect_send/recv

lib/async_req: make sure we return errors early from async_connect_send/recv

While it is true that [e]poll() only needs POLLOUT
and POLLERR/POLLHUP are added implicitly.
For tevent we need TEVENT_FD_READ in order to see POLLERR/POLLHUP.

The socket becomes only readable when we hit an error.
Waiting for TEVENT_FD_WRITE is needed for getting success,
while TEVENT_FD_READ is required to get failures.

This matches what we have in tstream_bsd_connect_send().

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>