Add a periodic script to backup output generated from zfs list, zfs get,
zpool list, and zpool get commands.

Disabled by default.

PR: 86388
Submitted by: Miroslav Lachman <000.fbsd@quip.cz>
Reviewed by: allanjude, 0mp
Approved by: allanjude (mentor)
MFC after: 4 weeks
Event: July 2020 Bugathon
Differential Revision: https://reviews.freebsd.org/D25638