Add the kernel build glue for the experimental NFS subsystem that
includes support for NFSv4. The subsystem can optionally be linked
into the kernel using the two options:

NFSCL - the client
NFSD - the server

It is also built as three modules:

nfscl - the client
nfsd - the server
nfscommon - functions shared by the client and server

Approved by: kib (mentor)