autovacuum integration attempt #3

Ok, here is a new patch for to integrate pg_autovacuum into the backend.

This patch adds the following to that last patch:
* updated to latest CVS
* changed GUC vars from autovac_* to autovacuum_*
* changed autovac_enabled GUC variable to autovacuum
* changed autovacuum GUC variable default to false
* improved autovacuum failure when row level stats aren't enabled

As before, this patch requires pg_autovacuum.c and .h get moved
from contrib to src/backend/postmaster and src/include/postmaster
respectively. In addition, the attached pg_autovacuum.h file must be put
in src/include/catelog/ for the new pg_autovacuum system
table.

Please apply to CVS or tell me what I need to change to get it applied.

Matthew