pg_autovacuum integration patch

Ok, I have an 1st cut patch for pg_autovacuum integration into the
backend. Please apply this patch to CVS or at least review and let me
know what I need to change to get it applied to CVS.

This patch requires that pg_autovacuum.c and .h are moved from contrib
to src/backend/postmaster and src/include/postmaster respectively. I
have also attached the pg_autovacuum.h file that needs to be put in
src/include/catelog/ for the new pg_autovacuum system table.

There is more to do for pg_autovacuum but I would like to get this patch
included into CVS or at least get it rejected now so I can fix it before
July 1.

With this patch pg_autovacuum:
* is a postmaster subprocess modeled after the bgwriter
* uses elog for all output (I guessed at the appropriate elog levels)
* used a new GUC variable to enable and disable pg_autovacuum
* stores all it's volitile data in a new pg_autovacuum system table
* allows admin to set per table thresholds

Matthew O'Connor