pgsql: Fix autovacuum to avoid leaving non-permanent Xids in

Log Message:
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Fix autovacuum to avoid leaving non-permanent Xids in non-connectable
databases.

Apply to the 8.1 branch only, as the new 8.2 (and HEAD) coding does not have
this problem.

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/src/backend/postmaster:
        autovacuum.c (r1.5.2.6 -> r1.5.2.7)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c.diff?r1=1.5.2.6&r2=1.5.2.7)