In multiple cases errors may be reported only once by the host socket
implementation, but should persist for Windows sockets. These cases are
currently not handled by poll_socket().

poll_socket() also does not include logic for filtering out events when asyncs
are queued or alerted on the relevant socket.

Hence, instead of duplicating more logic, remove the logic already duplicated,
and just call sock_poll_event(), so that there is one central place where events
are translated.

Mark the currently active poll async with a special "pending" field so that
poll_socket() does not attempt to complete it before all sockets are checked.