Get rid of uio_XXX macros used for the Mac OS/X port.

The NFS code had a bunch of Mac OS/X accessor functions named uio_XXX
left over from the port to Mac OS/X. Since that port is long forgotten,
replace the calls with the code generated by the FreeBSD macros for these
in nfskpiport.h. This allows the macros to be deleted from nfskpiport.h
and I think makes the code more readable.

This patch should not result in any semantic change.