pgsql: Change the autovacuum launcher to read pg_database directly,

Log Message:
-----------
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

Modified Files:
--------------
pgsql/src/backend/postmaster:
autovacuum.c (r1.103 -> r1.104)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c?r1=1.103&r2=1.104)
postmaster.c (r1.593 -> r1.594)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c?r1=1.593&r2=1.594)
pgsql/src/backend/storage/lmgr:
proc.c (r1.208 -> r1.209)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/proc.c?r1=1.208&r2=1.209)
pgsql/src/backend/utils/init:
globals.c (r1.109 -> r1.110)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/globals.c?r1=1.109&r2=1.110)
postinit.c (r1.195 -> r1.196)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/postinit.c?r1=1.195&r2=1.196)
pgsql/src/backend/utils/misc:
guc.c (r1.513 -> r1.514)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.513&r2=1.514)
pgsql/src/include/postmaster:
autovacuum.h (r1.15 -> r1.16)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/postmaster/autovacuum.h?r1=1.15&r2=1.16)
pgsql/src/include/storage:
proc.h (r1.113 -> r1.114)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/proc.h?r1=1.113&r2=1.114)