Add a new function called nfsm_uiombuflist(), similar to nfsm_uiombuf().

This patch adds a new function called nfsm_uiombuflist(), which is
similar to nfsm_uiombuf(), but doesn't not use the fields in
struct nfsrv_descript. This new function will be used by the pNFS client
for writing to mirrors using Flex Files layout.
The function is not yet called anywhere.
Also, get rid of #ifndef APPLE, which is ancient cruft left over from
the Mac OSX port of the NFSv4 client.