makefs: Handle missing link keyword in mtree spec
PR: 241178
Reviewed by: emaste
Pull Request: https://github.com/freebsd/freebsd-src/pull/850
(cherry picked from commit d26c1a0f8bceb6748436fe3fc57bac3ef8179dd3)
(cherry picked from commit be31bd59895e57fb7359de870ea3ecc19e3d1ae1)