pg_autovacuum should allow NULL values

I just tried to add something to the pg_autovacuum table for the first
time today (with 8.1). I wanted to make the simplest possible entry:
Disable auto-vacuuming for a table. However, the data model requires
that I also enter values for:

vac_base_thresh
vac_scale_factor
anl_base_thres
anl_scale_factor
vac_cost_delay
vac_cost_limit

None of those values matter when vacuuming is disabled for the table! I
suggest all these fields be nullable, and default to global values if
they are NULL.

These are "guts" and I should have to learn about them or fake them if I
just want to disable vacuuming for a table.

Likewise, if I just want to set one of the values, I shouldn't have to
set /all/ of them if the defaults are otherwise reasonable.

For the moment, I suppose I'll go and fake all these values so I can
disable a table from Vacuuming.

  Mark