BUG #1885: SHOW autovacuum settings tab completion broken

The following bug has been logged online:

Bug reference:      1885
Logged by:          Robert Treat
Email address:      xzilla@users.sourceforge.net
PostgreSQL version: 8.1beta1
Operating system:   FC2
Description:        SHOW autovacuum settings tab completion broken
Details:

When doing SHOW <tab><tab> or SHOW a<tab><tab> none of the autovacuum
settings come up.  I don't see anything in cvs head tab-complete.c that
stands out as being wrong, although it looks in psql like the SHOW list and
SET lists are being returned as the same thing, even though they should be
different according to the code.  Confirmed with someone on IRC that thier
autovacuum guc's didnt come up either.