Make deparsing of column defaults faster

The user docs say about column defaults: "The value is any variable-free
expression (subqueries and cross-references to other columns in the current
table are not allowed)"

And also say about pg_get_expr "If the expression might contain Vars,
specify the OID of the relation they refer to as the second parameter; if
no Vars are expected, zero is sufficient"

Since defaults can't contain Vars, this patch converts the second parameter
to zero in places where pg_get_expr is invoked on column defaults. Doing
this drops the time needed to run `pg_dump -s` on a 1600 column table with
defaults on all columns by a factor of 40. So pg_upgrade users with such
tables should see a big win (cc Justin).

I also made the change to the other places in the source which I could
identify with this issue. \d is now much faster for such tables as well,
but I didn't test the performance of the others.

There are surely more opportunities to speed up pg_get_expr for things
other than column defaults, but I wanted to submit this for now.

I have not tested the pg_upgrade using source versions <8.4, because I
can't get anything that old to "make check" correctly, so I have nothing to
test on.

Cheers,

Jeff