Re: cataloguing NOT NULL constraints

Here's v5.  I removed the business of renaming constraints in child
relations: recursing now just relies on matching column names.  Each
column has only one NOT NULL constraint; if you try to add another,
nothing happens.  All in all, this code is pretty similar to how we
handle inheritance of columns, which I think is good.

I added a mention that this funny syntax
  ALTER TABLE tab ADD CONSTRAINT NOT NULL col;
is not standard.  Maybe it's OK, but it seems a bit too prominent to me.

-- 
Álvaro Herrera        Breisgau, Deutschland  —  https://www.EnterpriseDB.com/