rsync backend

[NicholasBHubbard]

This PR is for work on the rsync backend

[NicholasBHubbard]

From the documentation of rsnapshot. This gives us some guidance on default options:

List of short arguments to pass to rsync. If not specified, "-a" is the default. Please note that these must be all next to each other. For example, "-az" is valid, while "-a -z" is not.

"-a" is rsync's "archive mode" which tells it to copy as much of the filesystem metadata as it can for each file. This specifically does not include information about hard links, as that would greatly increase rsync's memory usage and slow it down. If you need to preserve hard links in your backups, then add "H" to this.

List of long arguments to pass to rsync. The default values are --delete --numeric-ids --relative --delete-excluded. This means that the directory structure in each backup point destination will match that in the backup point source.

Quotes are permitted in rsync_long_args, eg --rsync-path='sudo /usr/bin/rsync'. You may use either single (') or double (") quotes, but nested quotes (including mixed nested quotes) are not permitted. Similar quoting is also allowed in per-backup-point rsync_long_args.