Fix ALTER DOMAIN NOT NULL syntax

This addresses a few problems with commit e5da0fe3c2 ("Catalog domain
not-null constraints").

In CREATE DOMAIN, a NOT NULL constraint looks like

    CREATE DOMAIN d1 AS int [ CONSTRAINT conname ] NOT NULL

(Before e5da0fe3c2, the constraint name was accepted but ignored.)

But in ALTER DOMAIN, a NOT NULL constraint looks like

    ALTER DOMAIN d1 ADD [ CONSTRAINT conname ] NOT NULL VALUE

where VALUE is where for a table constraint the column name would be.
(This works as of e5da0fe3c2.  Before e5da0fe3c2, this syntax
resulted in an internal error.)

But for domains, this latter syntax is confusing and needlessly
inconsistent between CREATE and ALTER.  So this changes it to just

    ALTER DOMAIN d1 ADD [ CONSTRAINT conname ] NOT NULL

(None of these syntaxes are per SQL standard; we are just living with
the bits of inconsistency that have built up over time.)

In passing, this also changes the psql \dD output to not show not-null
constraints in the column "Check", since it's already shown in the
column "Nullable".  This has also been off since e5da0fe3c2.

Reviewed-by: jian he <jian.universality@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/9ec24d7b-633d-463a-84c6-7acff769c9e8%40eisentraut.org

src/backend/utils/adt/ruleutils.c

@@ -2523,7 +2523,7 @@ pg_get_constraintdef_worker(Oid constraintId, bool fullCommand,
 				else if (conForm->contypid)
 				{
 					/* conkey is null for domain not-null constraints */
-					appendStringInfoString(&buf, "NOT NULL VALUE");
+					appendStringInfoString(&buf, "NOT NULL");
 				}
 				break;
 			}