pgsql: Improve autovacuum launcher's ability to detect a problem in

Log Message:
-----------
Improve autovacuum launcher's ability to detect a problem in worker startup,
by having the postmaster signal it when certain failures occur.  This requires
the postmaster setting a flag in shared memory, but should be as safe as the
pmsignal.c code is.

Also make sure the launcher honor's a postgresql.conf change turning it off
on SIGHUP.

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        autovacuum.c (r1.50 -> r1.51)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c.diff?r1=1.50&r2=1.51)
        postmaster.c (r1.527 -> r1.528)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.527&r2=1.528)
    pgsql/src/include/postmaster:
        autovacuum.h (r1.10 -> r1.11)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/postmaster/autovacuum.h.diff?r1=1.10&r2=1.11)