pgsql timezone support

Same problem as #279051: pgsql named time zone detection is incorrect and #323930: db_tz_support doesn't check Postgresql correctly

1. db_tz_support() test is invalid:

   $test = db_result(db_query("'2008-02-15 12:00:00 UTC' AT TIME ZONE 'US/Central'"));

   Should be:

   $test = db_result(db_query("SELECT '2008-02-15 12:00:00 UTC' AT TIME ZONE 'US/Central'"));

2. After making #1 fix above, exposes this bug in sql_tz()

   return "TIMESTAMP WITH TIME ZONE $field AT TIME ZONE $localzone"; ?>

   Is not valid (at least in postgres 8.1.11), since that syntax seems to work only with literals. This should be:

   return "$field::timestamp with time zone AT TIME ZONE $localzone";

Patch attached.