Fix ALTER TABLE ADD COLUMN ... PRIMARY KEY so that the new column is correctly

checked to see if it's been initialized to all non-nulls.  The implicit NOT
NULL constraint was not being checked during the ALTER (in fact, not even if
there was an explicit NOT NULL too), because ATExecAddColumn neglected to
set the flag needed to make the test happen.  This has been broken since
the capability was first added, in 8.0.

Brendan Jurd, per a report from Kaloyan Iliev.