Revert "Use pselect(2) not select(2), if available, to wait in postmaster's loop."

Revert "Use pselect(2) not select(2), if available, to wait in postmaster's loop."

This reverts commit b9515b62879722e3497236f6e0d6783c3fc059a2.

Buildfarm results suggest that some platforms have versions of pselect(2)
that are not merely non-atomic, but flat out non-functional.  Revert the
use-pselect patch to confirm this diagnosis (and exclude the no-SA_RESTART
patch as the source of trouble).  If it's so, we should probably look into
blacklisting specific platforms that have broken pselect.

Discussion: https://postgr.es/m/9696.1493072081@sss.pgh.pa.us