mitigations.7: move SSP documentation from security.7 to here

Stack Smashing Protection (SSP) is a software vulnerability mitigation,
and fits with this page. Add a note to the beginning of security.7
providing a more explicit cross reference to mitigations.7.

Reviewed by: kevans
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45435