bsd.man.mk: Add checkmanlinks target

This target checks that every page mentioned in MLINKS exists and that
every link corresponds to a (possibly commented-out) name entry in the
page it links to.

MFC after: 1 week
Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D51862

(cherry picked from commit 18a9f83630a7d4612066bb690bcec0437c0e2e72)