autovacuum launcher using InitPostgres

Hi,

This seems to be the last holdup for flatfiles.c.  This patch removes
usage of that code in autovacuum launcher, instead making it go through
the most basic relcache initialization so that it is able to read
pg_database.

To this end, InitPostgres has been split in two.  The launcher only
calls the first half, the rest of the callers have been patched to
invoke the second half.

The only thing I'm aware is missing from this patch is fixing up
avlauncher's signal handling, and adding a bit more commentary; also I
haven't tested it under EXEC_BACKEND yet.

Comments?

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support