Cause ALTER TABLE to perform ALTER COLUMN DROP DEFAULT operations during

Cause ALTER TABLE to perform ALTER COLUMN DROP DEFAULT operations during
the DROP pass rather than the ADD_CONSTR pass.  On examining the code I
think this was just an oversight rather than intentional, and it seems
to satisfy the principle of least surprise better than the alternative
solution that was discussed.  Add an example to the ref page showing how
to do ALTER TYPE and update the default in one command.  Per gripe from
Markus Bertheau that that wasn't possible.