Remove arbitrary ALTER TABLE .. ADD COLUMN restriction.

Remove arbitrary ALTER TABLE .. ADD COLUMN restriction.

The previous coding prevented ALTER TABLE .. ADD COLUMN from being used
with a non-NULL default in situations where the table's rowtype was being
used elsewhere.  But this is a completely arbitrary restriction since
you could do the same operation in multiple steps (add the column, add
the default, update the table).

Inspired by a patch from Noah Misch, though I didn't use his code.