pgsql: Make backends that are reading the pgstats file verify each

Log Message:
-----------
Make backends that are reading the pgstats file verify each backend PID
against the PGPROC array. Anything in the file that isn't in PGPROC
gets rejected as being a stale entry. This should solve complaints about
stale entries in pg_stat_activity after a BETERM message has been dropped
due to overload.

Modified Files:
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pgsql/src/backend/postmaster:
pgstat.c (r1.103 -> r1.104)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.103&r2=1.104)