need more ALTER TABLE guards for typed tables

After some investigation I figured that I need to add two more checks
into the ALTER TABLE code to prevent certain types of direct changes to
typed tables (see attached patch).

But it's not clear to me whether such checks should go into the "Prep"
or the "Exec" phases.  Prep seems more plausible to me, but some
commands such as DropColumn don't have a Prep handler.  A clarification
would be helpful.