pg_autovacuum (pg_avd version 2)

I have updated my pg_autovacuum program (formerly pg_avd, the name
changed as per discussion on the patches list).

This version should be a good bit better.  It addresses all the issues
pointed out by Neil Conway. Vacuum and Analyze are now handled
separately.  It now monitors for xid wraparound.  The number of database
connections and queries has been significantly reduced compared the
previous version.  I have moved it from bin to contrib.  More detail on
the changes are in the TODO file.

I have not tested the xid wraparound code as I have to let my AthlonXP
1600 run select 1 in a tight loop for approx. two days in order to
perform the required 500,000,000 xacts.

Side note: in trying to test the xid wrap around code I noted that the
select 1 query is nearly 30% slower with stats_row_level = true.  I
guess this is another reason for a backend integrated autovacuum that
not built on top of the stats system.  I know the net effect is reduced
with more realistic queries but still seems like a lot.

As always, any and all feedback is appreciated.

Matthew T. O'Connor