sysctl(8): Mention more security.bsd knobs; Refer to security(7)

Approved by: re (gjb)
Reviewed by: mhorne, pauamma_gundo.com, emaste
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D41113

(cherry picked from commit 8d7a48d367ffde2a29419ef943c4099984e3af4d)
(cherry picked from commit 0e480eda041122ec8571d14ed1ebe8787a384972)