pg_autovacuum integration attempt #2

I have make the changes suggested after I posted my last patch, I have
also make several additional improvements. it needs to be tested more,
but since the deadline is coming up so soon, I wanted to post an update
just to keep everyone abreast of what I'm doing. Please review and let
me know what I need to change to get it applied to CVS.

As before, 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.

I assume I will have to write the docs for this, but right now I'm
focusing on the code, I get get the docs written after the feature
freeze.

Matthew O'Connor