The last user of this feature has been removed, so delete the feature too,
simplifying poll-loop.c significantly.
poll_cancel() is no longer used, either, but deleting it is much less
beneficial.
Rather than running signal hooks directly from the actual signal
handler, simply record the fact that the signal occured and run
the hook next time around the poll loop. This allows significantly
more freedom as to what can actually be done in the signal hooks.
