Fix assorted inconsistencies in GIN opclass support function declarations.

Fix assorted inconsistencies in GIN opclass support function declarations.

GIN had some minor issues too, mostly using "internal" where something
else would be more appropriate.  I went with the same approach as in
9ff60273e35cad6e, namely preferring the opclass' indexed datatype for
arguments that receive an operator RHS value, even if that's not
necessarily what they really are.

Again, this is with an eye to having a uniform rule for ginvalidate()
to check support function signatures.