Add pselect implementation for platforms without.

Add pselect implementation for platforms without.

This is basically the existing notify_pipe kludge from serverloop.c
moved behind a pselect interface.  It works by installing a signal
handler that writes to a pipe that the select is watching, then calls
the original handler.

The select call in serverloop will become pselect soon, at which point the
kludge will be removed from thereand will only exist in the compat layer.
Original code by markus, help from djm.