Add mitigations(7) describing our vulnerability mitigations

This is an initial take on documenting vulnerability mitigations.

Connect mitigations(7) to the build

Also add some cross references.

Reviewed by: gbe (earlier)
Sponsored by: The FreeBSD Foundation
Co-authored-by: Olivier Certner <olce.freebsd@certner.fr>
Differential Revision: https://reviews.freebsd.org/D41794

(cherry picked from commit b6a61ac2d475fba9c45f7f407549a522f60dea18)
(cherry picked from commit 6e5dcc6113da649a79e5bc2c3ea9329bcd1d85d5)
(cherry picked from commit a2b289608d6b54b6e1ce85096560807e551bb5ac)