r5152: Restructure the directory handling code, stop using void * pointers

r5152: Restructure the directory handling code, stop using void * pointers
that just allow the wrong pointer to be assigned :-) and make the
interface more consistent. Fix the FreeBSD directory problem. Last
thing to do is to add the "singleton" directory concept from James
Peach's code.
Jeremy.
(This used to be commit cfa8150fd9932470cb8f3b5e14c0156dda67125d)