Committed patches; initdb required

I've committed patches to do the following:

o Support AT TIME ZONE date/time syntax.

o Fix INTERVAL mixed-sign output representation. "ISO format" has
changed a little, and no longer uses the "AGO" qualifier but instead
uses signed numbers.

o Allow functions called with untyped constant strings to fall back to a
string type if available. Previously, PostgreSQL refused to chose
between types from different categories (e.g. string vs numeric).

o Add tests for OUTER JOIN syntax to the regression test. Needs more
work.

Since a couple of functions were added to pg_proc, an initdb is required
(sorry!).

- Thomas