Replace .h files included from old NFS directories with the new NFS ones.

Prior to this patch, nfsstat.c includes files from sys/nfs, sys/nfsclient
and sys/nfsserver. These .h files (particularily the ones in sys/nfsclient
and sys/nfsserver) are from the old NFS code and should eventually be
deprecated/removed.
This patch changes nfsstat.c to include files from the new/current NFS
code instead of the old ones in preparation for eventual removal.