pgsql: Force VACUUM to recalculate oldestXmin even when we haven't

Log Message:
-----------
Force VACUUM to recalculate oldestXmin even when we haven't changed our
own database's datfrozenxid, if the current value is old enough to be
forcing autovacuums or warning messages. This ensures that a bogus
value is replaced as soon as possible. Per a comment from Heikki.

Modified Files:
--------------
pgsql/src/backend/access/transam:
varsup.c (r1.85 -> r1.86)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/varsup.c?r1=1.85&r2=1.86)
pgsql/src/backend/commands:
vacuum.c (r1.392 -> r1.393)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c?r1=1.392&r2=1.393)
pgsql/src/include/access:
transam.h (r1.69 -> r1.70)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/transam.h?r1=1.69&r2=1.70)