Re: Support NOT VALID / VALIDATE constraint options for named NOT NULL constraints

It occurred to me that we will also want to have NOT NULL NOT ENFORCED 
constraints eventually.  As we have discussed elsewhere, the NOT 
ENFORCED state is closely related to the NOT VALID state.  So that 
should probably be considered in the design here.

Reading up on this again now, I'm confused about putting the NOT VALID 
state for not-null constraints into pg_attribute.  We have catalogued 
not-null constraints now, so we can put metadata for them into 
pg_constraint!  And we have NOT VALID and NOT ENFORCED flags in 
pg_constraint already.

So what is the purpose of the attnotnullvalid field?  In the latest 
posted patch, I don't see this column used in the executor for the 
actual constraint checking.  So is this all merely for clients to 
understand the constraint metadata?  If we add more metadata for 
not-null constraints, do we need to add a new pg_attribute flag for each 
one?  That doesn't seem right.