Clean up support for FreeBSD 11.2.

While here, modernize some comments in Mk/bsd.*.mk.

Note that graphics/drm-fbsd11.2-kmod is not renamed yet, this was somewhat
under discussion.

Submitted by: rene
Reviewed by: antoine, jbeich, mat, zeising
Differential Revision: https://reviews.freebsd.org/D21974