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 81069a9efc5a374dd39874a161f456f0fb3afba4.

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