mac(4): update the references to MAC modules

Add entries for mac_ntpd(4) and mac_priority(4) to the table of MAC
modules.

Drop the entry for mac_none(4) from the list, but retain the
cross-reference in SEE ALSO. This module has no functional impact and is
of minimal interest to users. Add a new cross-reference to the similar
mac_stub(4), limited to SEE ALSO for the same reasoning.

Reviewed by: Pau Amma <pauamma@gundo.com>
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40483

(cherry picked from commit e2cb7201015dbd2d4cea5c9b4e0385d11614263c)