Improve autovacuum launcher's ability to detect a problem in worker startup,

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.