small psql patch - show Schema name for \dt \dv \dS

I can't remember who said they were working on schema related psql
changes, but I kept finding myself wishing I could see what schema a
table or view exists in when I use \dt, \dv, etc. So, here is a patch
which does just that.

It sorts on "Schema" first, and "Name" second.

It also changes the test for system objects to key off the namespace
name starting with 'pg_' instead of the object name.

Sample output:

test=# create schema testschema;
CREATE SCHEMA
test=# create view testschema.ts_view as select 1;
CREATE VIEW
test=# \dv
List of relations
Name | Schema | Type | Owner
--------------------+------------+------+----------
__testpassbyval | public | view | postgres
fooview | public | view | postgres
master_pg_proc | public | view | postgres
rmt_pg_proc | public | view | postgres
vw_dblink_get_pkey | public | view | postgres
vw_dblink_replace | public | view | postgres
ts_view | testschema | view | postgres
(7 rows)

If there are no objections, please apply.

Thanks!

Joe