Show sequences owned by

The attached patch makes the \d output for psql on a sequence show
which table/column owns the sequence. The table already showed the
dependency the other way through the default value, but going from
sequence back to table was not possible.

Comments/reviews?

-- 
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/