Always install nmtree as mtree.

For compability, link mtree to nmtree.
Move mtree.5 to usr.sbin/nmtree.

Remove note that mtree 2.0 format files aren't supported.
Add WITHOUT_FMTREE to disable building fmtree.
Now that mtree is always nmtree use it as mtree.
Document WITHOUT_NCURSESW.