BUG/MINOR: signals/poller: ensure wakeup from signals

BUG/MINOR: signals/poller: ensure wakeup from signals

Add self-wake in signal_handler() to fix a race condition with a signal
coming in between checking signal_queue_len and entering polling sleep.

The changes in commit 43c891dda ("BUG/MINOR: signals/poller: set the
poller timeout to 0 when there are signals") were insufficient.

Move the signal_queue_len check from the poll implementations to
run_poll_loop() to keep that logic in one place.

The poll loops are terminated either by the parameter wake being set or
wake up due to a write to their poller_wr_pipe by wake_thread() in
signal_handler().

This fixes issue #1841.

Must be backported in every stable version.