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.