autovauum integration patch: Attempt #4

Ok, sorry about the mis-fire on the last patch I submitted, this one is
what I was meant to send.

As before:

This patch adds the following since attempt #2:
* 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, to apply this patch:
1) Move pg_autovacuum.c and .h get from contrib to
src/backend/postmaster and src/include/postmaster respectively.
2) Place the attached pg_autovacuum.h in src/include/catelog/
3) Apply the attached diff file

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

Thanks again,

Matthew