Make CREATE TYPE print warnings if a datatype's I/O functions are volatile.
This is a followup to commit
43ac12c6e6e397fd9142ed908447eba32d3785b2,
which added regression tests checking that I/O functions of built-in
types are not marked volatile. Complaining in CREATE TYPE should push
developers of add-on types to fix any misdeclared functions in their
types. It's just a warning not an error, to avoid creating upgrade
problems for what might be just cosmetic mis-markings.
Aside from adding the warning code, fix a number of types that were
sloppily created in the regression tests.