pgsql: Fix vacuum so that autovacuum is really not cancelled when doing

Log Message:
-----------
Fix vacuum so that autovacuum is really not cancelled when doing an emergency
job (i.e. to prevent Xid wraparound problems.) Bug reported by ITAGAKI
Takahiro in 20080314103837(dot)63D3(dot)52131E4D(at)oss(dot)ntt(dot)co(dot)jp, though I didn't use his
patch.

Modified Files:
--------------
pgsql/src/backend/commands:
vacuum.c (r1.366 -> r1.367)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c?r1=1.366&r2=1.367)
pgsql/src/backend/postmaster:
autovacuum.c (r1.72 -> r1.73)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c?r1=1.72&r2=1.73)
pgsql/src/backend/tcop:
utility.c (r1.289 -> r1.290)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c?r1=1.289&r2=1.290)
pgsql/src/include/commands:
vacuum.h (r1.75 -> r1.76)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/vacuum.h?r1=1.75&r2=1.76)