experimental nfs subsystem from sys/fs/nfs/nfs.h and sys/fs/nfs/nfsproto.h
to sys/fs/nfs/nfsport.h and rename nfsstat to ext_nfsstat. This was done
so that src/usr.bin/nfsstat.c could use it alongside the regular nfs
include files and struct nfsstat.
Approved by: kib (mentor)