Disallow direct change of NO INHERIT of not-null constraints

We support changing NO INHERIT constraint to INHERIT for constraints in
child relations when adding a constraint to some ancestor relation, and
also during pg_upgrade's schema restore; but other than those special
cases, command ALTER TABLE ADD CONSTRAINT should not be allowed to
change an existing constraint from NO INHERIT to INHERIT, as that would
require to process child relations so that they also acquire an
appropriate constraint, which we may not be in a position to do.  (It'd
also be surprising behavior.)

It is conceivable that we want to allow ALTER TABLE SET NOT NULL to make
such a change; but in that case some more code is needed to implement it
correctly, so for now I've made that throw the same error message.

Also, during the prep phase of ALTER TABLE ADD CONSTRAINT, acquire locks
on all descendant tables; otherwise we might operate on child tables on
which no locks are held, particularly in the mode where a primary key
causes not-null constraints to be created on children.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/7d923a66-55f0-3395-cd40-81c142b5448b@gmail.com

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