Change the autovacuum launcher to read pg_database directly, rather than

Change the autovacuum launcher to read pg_database directly, rather than
via the "flat files" facility.  This requires making it enough like a backend
to be able to run transactions; it's no longer an "auxiliary process" but
more like the autovacuum worker processes.  Also, its signal handling has
to be brought into line with backends/workers.  In particular, since it
now has to handle procsignal.c processing, the special autovac-launcher-only
signal conditions are moved to SIGUSR2.

Alvaro, with some cleanup from Tom