membarrier: Add manual page

Add a minimal membarrier man page that documents the available cmd
values and errors that can be returned. We can add more information and
iterate on it in the tree.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46967

(cherry picked from commit 1fc766e3b41d0cdbd166ef95258434069a90ca52)
(cherry picked from commit 92cd5abb64dd70c305535c9504c6a2b73552147f)
(cherry picked from commit 8b41e693fc3956385d5771d60ee93e18001a5a0d)