Treat EPERM as a non-error case when checking to see if old postmaster

Treat EPERM as a non-error case when checking to see if old postmaster
is still alive.  This improves our odds of not getting fooled by an
unrelated process when checking a stale lock file.  Other checks already
in place, plus one newly added in checkDataDir(), ensure that we cannot
attempt to usurp the place of a postmaster belonging to a different userid,
so there is no need to error out.  Add comments indicating the importance
of these other checks.