Reform rsbackup cron arrangements

[ewxrjk]

Currently rsbackup has cron scripts to arrange for backups of chosen hosts to happen daily, weekly, etc. It is controlled by a separate configuration from the main config file.

With the introduction of backup policies this configuration can be moved into the main configuration file. The part of rsbackup.cron which performs backups can be reduced to rsbackup --backup, and invoked as frequently as the operator desires. The default would be hourly. --wait would not be used, if a backup is in progress we will just try again in an hour.

That leaves:

• --prune. There is no reason this couldn't be done at the maximum frequency since there are already prune policies controlling what is pruned.
• --prune-incomplete. My current thinking is that this would be invoked daily, with --wait.
• --email. Again my current thinking is that this would be invoked daily, with --wait.

While on the subject of backup policies there should probably be an option to override backup policies and always backup the selected volumes.