Deliver hangup events to blocked pollers on unix socket shutdown

The shutdown notification paths only notified ReadableEvents and
WritableEvents, so a poller waiting only for POLLHUP or POLLRDHUP was
never woken and slept until its timeout. Linux wakes all waiters via
sk_state_change(). Notifications now include EventRdHUp and EventHUp
where the closed direction can newly satisfy them; over-notifying is
harmless since woken waiters re-check readiness.

PiperOrigin-RevId: 966685883