Remove Mac OS/X macros that did nothing for FreeBSD.

The macros CAST_USER_ADDR_T() and CAST_DOWN() were used for the Mac OS/X
port. The first of these macros was a no-op for FreeBSD and the second
is no longer used.
This patch gets rid of them. It also deletes the "mbuf_t" typedef which
is no longer used in the FreeBSD code from nfskpiport.h

This patch should not change semantics.