pgsql: Improve xid wraparound message (the server isn't really shut

Log Message:
-----------
Improve xid wraparound message (the server isn't really shut down, just
not accepting queries).

errmsg("database is not accepting queries to avoid
wraparound data loss in database \"%s\"",
errhint("Stop the postmaster and use a standalone
backend to VACUUM database \"%s\".",

Modified Files:
--------------
pgsql/src/backend/access/transam:
varsup.c (r1.65 -> r1.66)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/varsup.c.diff?r1=1.65&r2=1.66)