Fix pg_get_constraintdef for NOT NULL constraints on domains

We added pg_constraint rows for all not-null constraints, first for
tables and later for domains; but while the ones for tables were
reverted, the ones for domains were not.  However, we did accidentally
revert ruleutils.c support for the ones on domains in 6f8bb7c1e9,
which breaks running pg_get_constraintdef() on them.  Put that back.

This is only needed in branch 17, because we've reinstated this code in
branch master with commit 14e87ffa5c.  Add some new tests in both
branches.

I couldn't find anything else that needs de-reverting.

Reported-by: Erki Eessaar <erki.eessaar@taltech.ee>
Reviewed-by: Magnus Hagander <magnus@hagander.net>
Discussion: https://postgr.es/m/AS8PR01MB75110350415AAB8BBABBA1ECFE222@AS8PR01MB7511.eurprd01.prod.exchangelabs.com

src/test/regress/expected/domain.out

@@ -743,6 +743,20 @@ update domnotnull set col1 = null; -- fails
 ERROR:  domain dnotnulltest does not allow null values
 alter domain dnotnulltest drop not null;
 update domnotnull set col1 = null;
+update domnotnull set col1 = 5;
+-- these constraints can also be added and removed by name
+alter domain dnotnulltest add constraint dnotnulltest_notnull not null;
+update domnotnull set col1 = null;		-- fails
+ERROR:  domain dnotnulltest does not allow null values
+select conname, pg_get_constraintdef(oid) from pg_constraint
+ where contypid = 'dnotnulltest'::regtype;
+       conname        | pg_get_constraintdef 
+----------------------+----------------------
+ dnotnulltest_notnull | NOT NULL
+(1 row)
+
+alter domain dnotnulltest drop constraint dnotnulltest_notnull;
+update domnotnull set col1 = null;
 drop domain dnotnulltest cascade;
 NOTICE:  drop cascades to 2 other objects
 DETAIL:  drop cascades to column col2 of table domnotnull

src/test/regress/sql/domain.sql

@@ -429,6 +429,17 @@ alter domain dnotnulltest drop not null;
 
 update domnotnull set col1 = null;
 
+update domnotnull set col1 = 5;
+
+-- these constraints can also be added and removed by name
+alter domain dnotnulltest add constraint dnotnulltest_notnull not null;
+update domnotnull set col1 = null;		-- fails
+select conname, pg_get_constraintdef(oid) from pg_constraint
+ where contypid = 'dnotnulltest'::regtype;
+
+alter domain dnotnulltest drop constraint dnotnulltest_notnull;
+update domnotnull set col1 = null;
+
 drop domain dnotnulltest cascade;
 
 -- Test ALTER DOMAIN .. DEFAULT ..