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

Log Message:
-----------
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.

Modified Files:
--------------
pgsql/src/backend/postmaster:
postmaster.c (r1.446 -> r1.447)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.446&r2=1.447)
pgsql/src/backend/utils/init:
miscinit.c (r1.137 -> r1.138)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/miscinit.c.diff?r1=1.137&r2=1.138)