Fix psql \d's query for identifying parent triggers.

The original coding (from c33869cc3) failed with "more than one row
returned by a subquery used as an expression" if there were unrelated
triggers of the same tgname on parent partitioned tables.  (That's
possible because statement-level triggers don't get inherited.)  Fix
by applying LIMIT 1 after sorting the candidates by inheritance level.

Also, wrap the subquery in a CASE so that we don't have to execute it at
all when the trigger is visibly non-inherited.  Aside from saving some
cycles, this avoids the need for a confusing and undocumented NULLIF().

While here, tweak the format of the emitted query to look a bit
nicer for "psql -E", and add some explanation of this subquery,
because it badly needs it.

Report and patch by Justin Pryzby (with some editing by me).
Back-patch to v13 where the faulty code came in.

Discussion: https://postgr.es/m/20211217154356.GJ17618@telsasoft.com

src/test/regress/expected/triggers.out

@@ -2122,6 +2122,20 @@ Triggers:
 alter table trigpart attach partition trigpart3 FOR VALUES FROM (2000) to (3000); -- fail
 ERROR:  trigger "trg1" for relation "trigpart3" already exists
 drop table trigpart3;
+-- check display of unrelated triggers
+create trigger samename after delete on trigpart execute function trigger_nothing();
+create trigger samename after delete on trigpart1 execute function trigger_nothing();
+\d trigpart1
+             Table "public.trigpart1"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ a      | integer |           |          | 
+ b      | integer |           |          | 
+Partition of: trigpart FOR VALUES FROM (0) TO (1000)
+Triggers:
+    samename AFTER DELETE ON trigpart1 FOR EACH STATEMENT EXECUTE FUNCTION trigger_nothing()
+    trg1 AFTER INSERT ON trigpart1 FOR EACH ROW EXECUTE FUNCTION trigger_nothing(), ON TABLE trigpart
+
 drop table trigpart;
 drop function trigger_nothing();
 --

src/test/regress/sql/triggers.sql

@@ -1428,6 +1428,11 @@ create trigger trg1 after insert on trigpart3 for each row execute procedure tri
 alter table trigpart attach partition trigpart3 FOR VALUES FROM (2000) to (3000); -- fail
 drop table trigpart3;
 
+-- check display of unrelated triggers
+create trigger samename after delete on trigpart execute function trigger_nothing();
+create trigger samename after delete on trigpart1 execute function trigger_nothing();
+\d trigpart1
+
 drop table trigpart;
 drop function trigger_nothing();