This patch set applies the explicit catalog representation of not-null
constraints introduced by b0e96f3119 for table constraints also to
domain not-null constraints.
Since there is no inheritance or primary keys etc., this is much simpler
and just applies the existing infrastructure to domains as well. As a
result, domain not-null constraints now appear in the information schema
correctly. Another effect is that you can now use the ALTER DOMAIN ...
ADD/DROP CONSTRAINT syntax for not-null constraints as well. This makes
everything consistent overall.
For the most part, I structured the code so that there are now separate
sibling subroutines for domain check constraints and domain not-null
constraints. This seemed to work out best, but one could also consider
other ways to refactor this.
