Modify src/etc/mtree/BSD.include.dist and src/include/Makefile

Modify src/etc/mtree/BSD.include.dist and src/include/Makefile
so that the .h files in src/sys/fs/nfs will be installed under
/usr/include/fs/nfs. This will allow the following utilities to
build, once additions and changes for the experimental nfs subsystem
are committed:
usr.sbin/mountd - Once modified to add support for the
  experimental nfs subsystem.
ur.sbin/nfsstat - Once modified to add support for the
  experimental nfs subsystem.
usr.sbin/nfscbd - The client side callback daemon for NFSv4.
usr.sbin/nfsuserd - The NFSv4 user/group name<->uid/gid mapping daemon.
usr.sbin/nfsdumpstate - The NFSv4 utility for dumping open/lock state.
usr.sbin/nfsrevoke - The sysadmin command for revoking NFSv4 state.

Approved by:	kib (mentor)