Add -S option to veriexec

During software installation, use veriexec -S to strictly
enforce certificate validity checks (notBefore, notAfter).

Otherwise ignore certificate validity period.
It is generally unacceptible for the Internet to stop working
just because someone did not upgrade their infrastructure for a decade.

Sponsored by: Juniper Networks, Inc.

Reviewed by: sebastien.bini_stormshield.eu
Differential Revision: https://reviews.freebsd.org/D35758