Re: not null constraints, again

Here's another version where I do skip searching for children twice, and
rewrote the comments.

I also noticed that in child tables we were only looking for
pg_attribute.attnotnull, and not whether the constraints had been
validated or made inheritable.  This seemed a wasted opportunity, so I
refactored the code to instead examine the pg_constraint row and apply
the same checks as for the constraint on the parent (namely, that it's
valid and not NO INHERIT).  We already check for these things downstream
(alter table phase 2, during AdjustNotNullInheritance), but only after
potentially wasting more work, so it makes sense to do it here (alter
table phase 1) given that it's very easy.  I added some tests for these
things also, as those cases weren't covered.