Add a periodic script to backup output generated from gmirror list.

Disabled by default.

PR: 86388
Submitted by: Miroslav Lachman <000.fbsd@quip.cz>
Reviewed by: allanjude, gbe
Approved by: allanjude (mentor)
MFC after: 4 weeks
Event: July 2020 Bugathon
Differential Revision: https://reviews.freebsd.org/D25631