Re: cataloguing NOT NULL constraints

There were a lot more problems in that submission than I at first
realized, and I had to rewrite a lot of code in order to fix them. I
have fixed all the user-visible problems I found in this version, and
reviewed the tests results more carefully so I am now more confident
that behaviourally it's doing the right thing; but

1. the pg_upgrade test problem is still unaddressed,
2. I haven't verified that catalog contents is correct, especially
regarding dependencies,
3. there are way too many XXX and FIXME comments sprinkled everywhere.

I'm sure a couple of these XXX comments can be left for later work, and
there's a few that should be dealt with by merely removing them; but the
others (and all FIXMEs) represent pending work.

Also, I'm not at all happy about having this new ConstraintNotNull
artificial node there; perhaps this can be solved by using a regular
Constraint with some new flag, or maybe it will even work without any
extra flags by the fact that the node appears where it appears. Anyway,
requires investigation. Also, the AT_SetAttNotNull continues to irk me.

test_ddl_deparse is also unhappy. This is probably an easy fix;
apparently, ATExecDropConstraint has been doing things wrong forever.

Anyway, here's version 2 of this, with apologies for those who spent
time reviewing version 1 with all its brokenness.

--
Álvaro Herrera PostgreSQL Developer — https://www.EnterpriseDB.com/
"On the other flipper, one wrong move and we're Fatal Exceptions"
(T.U.X.: Term Unit X - http://www.thelinuxreview.com/TUX/)