Run the postmaster's signal handlers without SA_RESTART.

Run the postmaster's signal handlers without SA_RESTART.

The postmaster keeps signals blocked everywhere except while waiting
for something to happen in ServerLoop().  The code expects that the
select(2) will be cancelled with EINTR if an interrupt occurs; without
that, followup actions that should be performed by ServerLoop() itself
will be delayed.  However, some platforms interpret the SA_RESTART
signal flag as meaning that they should restart rather than cancel
the select(2).  Worse yet, some of them restart it with the original
timeout delay, meaning that a steady stream of signal interrupts can
prevent ServerLoop() from iterating at all if there are no incoming
connection requests.

Observable symptoms of this, on an affected platform such as HPUX 10,
include extremely slow parallel query startup (possibly as much as
30 seconds) and failure to update timestamps on the postmaster's sockets
and lockfiles when no new connections arrive for a long time.

We can fix this by running the postmaster's signal handlers without
SA_RESTART.  That would be quite a scary change if the range of code
where signals are accepted weren't so tiny, but as it is, it seems
safe enough.  (Note that postmaster children do, and must, reset all
the handlers before unblocking signals; so this change should not
affect any child process.)

There is talk of rewriting the postmaster to use a WaitEventSet and
not do signal response work in signal handlers, at which point it might
be appropriate to revert this patch.  But that's not happening before
v11 at the earliest.

Back-patch to 9.6.  The problem exists much further back, but the
worst symptom arises only in connection with parallel query, so it
does not seem worth taking any portability risks in older branches.

Discussion: https://postgr.es/m/9205.1492833041@sss.pgh.pa.us