Require superuser privilege to create base types (but not composites, enums,

Require superuser privilege to create base types (but not composites, enums,
or domains).  This was already effectively required because you had to own
the I/O functions, and the I/O functions pretty much have to be written in
C since we don't let PL functions take or return cstring.  But given the
possible security consequences of a malicious type definition, it seems
prudent to enforce superuser requirement directly.  Per recent discussion.