Add a periodic script to backup the partition table and boot code

Optionally, alert you if the contents change from the previous backup

PR: 86388
Submitted by: Rob Fairbanks <rob.fx907@gmail.com>, Miroslav Lachman <000.fbsd@quip.cz> (Original Version)
MFC after: 4 weeks
Relnotes: yes
Sponsored by: Klara Inc.
Event: July 2020 Bugathon
Differential Revision: https://reviews.freebsd.org/D25628