Revert commit ab0f7b60 (in HEAD only)
in favor of the proper solution, which is to declare enum_recv() correctly
in the system catalogs.  It should be declared to take type "internal"
not "cstring".

Also improve the type_sanity regression test, which should have caught
this typo, so that it actually would.  Most of the relevant checks on
the signature of type I/O functions should not have been restricted to
basetypes/pseudotypes, as they should apply to any type's I/O functions.