Believe it or not, "create_type" has never, ever checked to see if the
given supertype is a valid type. The code now verifies that the given
supertype is a child of "acs_object", except when supertype is passed in
as NULL or '' (in which case it's set to acs_object). Only did it for
PostgreSQL since it's been around forever and whatever Oracle users don't
know, won't hurt them.