MAC: mac.h: Separately test inclusion from userspace

Reviewed by: jamie
Approved by: markj (mentor)
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46897

(cherry picked from commit b47f4718c0725f4ea2e8f88359e86661e8dc4a38)