pgsql: Fix some sloppiness in the recent multiple-autovacuum-worker

Log Message:
-----------
Fix some sloppiness in the recent multiple-autovacuum-worker patch.  It was
not bothering to initialize is_autovacuum for regular backends, meaning there
was a significant chance of the postmaster prematurely sending them SIGTERM
during database shutdown.  Also, leaving the cancel key unset for an autovac
worker meant that any client could send it SIGINT, which doesn't sound
especially good either.

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        postmaster.c (r1.537 -> r1.538)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.537&r2=1.538)