pgsql: Treat the autovac launcher more like a regular backend, in that

Log Message:
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Treat the autovac launcher more like a regular backend, in that we wait
for it to die before telling the bgwriter to initiate shutdown checkpoint.
Since it's connected to shared memory, this seems more prudent than the
alternative of letting it quit asynchronously.  Resolves my complaint
of yesterday about repeated shutdown checkpoints in CVS HEAD.

Modified Files:
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    pgsql/src/backend/postmaster:
        postmaster.c (r1.529 -> r1.530)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.529&r2=1.530)