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

Reviewed by: mhorne, pauamma_gundo.com, emaste
MFC after: 2 weeks
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D41113

(cherry picked from commit 8d7a48d367ffde2a29419ef943c4099984e3af4d)