rsnapshot: New addon

rsnapshot: New addon

- What is it?
	rsnapshot is a filesystem snapshot utility based on
	rsync. rsnapshot makes it easy to make periodic snapshots of the
	ipfire device. The code makes extensive use of hard links whenever
	possible, to greatly reduce the disk space required.  See:
	https://rsnapshot.org

- Why is it needed?
	Rsnapshot backups run multiple times per day
	(e.g., once per day up to 24 times per day). Rsnapshot is much easier
	to configure, setup and use than the borg backup add-on.  (I found
	borg somewhat confusing). Rsnapshot completes each backup very fast.
	Unlike borg, rsnapshot does not compress each backup before storage.
	During a complete rebuild, borg backup need installation of the borg
	add-on to recover archived files.  Rsnapshot backups can be copied
	directly from the backup drive. Current backups (backup.pl or borg)
	could corrupt sqlite3 databases by running a backup during a database
	write.  This add-on includes a script specifically for sqlite backups.

- IPFire Wiki
	In process at: https://wiki.ipfire.org/addons/rsnapshot

Thanks to Gerd for creating a first build and a nice template for me!

Signed-off-by: Jon Murphy <jon.murphy@ipfire.org>