bsd.man.mk: Support MANGROUPS.yes

Similar to SUBDIR and CONFGROUPS, this allows the use of
MANGROUPS.${MK_FOO}, simplifying Makefile logic for optional
manpages.

MFC after: 3 seconds
Reviewed by: emaste
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53126

(cherry picked from commit e757a92282fcd064c82cf2685dde4730743fc425)