Re: cataloguing NOT NULL constraints

I have now pushed this again.  Hopefully it'll stick this time.

We may want to make some further tweaks to the behavior in some cases --
for example, don't disallow ALTER TABLE DROP NOT NULL when the
constraint is both inherited and has a local definition; the other
option is to mark the constraint as no longer having a local definition.
I left it the other way because that's what CHECK does; maybe we would
like to change both at once.

I ran it through CI, and the pg_upgrade test with a dump from 14's
regression test database and everything worked well, but it's been a
while since I tested the sepgsql part of it, so that might the first
thing to explode.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/