Allow psql's \dt and \di to show TOAST tables and their indexes.

Formerly, TOAST objects were unconditionally suppressed, but since
\d is able to print them it's not very clear why these variants
should not.  Instead, use the same rules as for system catalogs:
they can be seen if you write the 'S' modifier or a table name
pattern.  (In practice, since hardly anybody would keep pg_toast
in their search_path, it's really down to whether you use a pattern
that can match pg_toast.*.)

No docs change seems necessary because the docs already say that
this happens for "system objects"; we're just classifying TOAST
tables as being that.

Justin Pryzby, reviewed by Laurenz Albe

Discussion: https://postgr.es/m/20201130165436.GX24052@telsasoft.com

src/test/regress/sql/psql.sql

@@ -1210,6 +1210,8 @@ drop role regress_partitioning_role;
 
 -- \d on toast table (use pg_statistic's toast table, which has a known name)
 \d pg_toast.pg_toast_2619
+\dt pg_toast.pg_toast_2619
+\di pg_toast.pg_toast_2619_index
 
 -- check printing info about access methods
 \dA