src/c-types-impl.lisp: Handle `void' argument lists specially.

src/c-types-impl.lisp: Handle `void' argument lists specially.

We'll treat them as equivalent to empty lists.  Otherwise it's just too
easy to declare a message as

	void msg(void)

and end up with a horrible mess when the translator tries to turn that
into a direct method of the form

	void msg(Thing *me, void)

which works less then brilliantly.

And, on output, we'll write an empty list as `void'.  This situation
doesn't arise in the current translator (I think), but might well do in
extensions.