pgsql: Fix DecodeDateTime to allow timezone to appear before year.

Log Message:
-----------
Fix DecodeDateTime to allow timezone to appear before year. This had
historically worked in some but not all cases, but as of 8.2 it failed for all
timezone formats. Fix, and add regression test cases to catch future
regressions in this area. Per gripe from Adam Witney.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
datetime.c (r1.180 -> r1.181)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c.diff?r1=1.180&r2=1.181)
pgsql/src/test/regress/expected:
timestamptz.out (r1.26 -> r1.27)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/timestamptz.out.diff?r1=1.26&r2=1.27)
pgsql/src/test/regress/sql:
timestamptz.sql (r1.14 -> r1.15)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/timestamptz.sql.diff?r1=1.14&r2=1.15)