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.

Tags:
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REL8_0_STABLE

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        postmaster.c (r1.443 -> r1.443.4.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.443&r2=1.443.4.1)
    pgsql/src/backend/utils/init:
        miscinit.c (r1.137 -> r1.137.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/miscinit.c.diff?r1=1.137&r2=1.137.4.1)