backup server support and incremental backups

[kilburn]

On the one hand, website backups (files+db, stored in /backups inside the users' folders) are the clients' responsiblity. *If* they want such backups, the space should count. They can always download them and free this space.

On the other hand, server backups (emergency recovery backups, just in case something really bad happens to the server) are the provider's responsiblity, yet such responsibility might be explicitly rejected in the service contract.

We could try to unify both types of backups, but I think it is a bad idea. Per-client backups are something that should clearly be managed by the server, whereas full-server backups should not because there are *many* strategies to do it.

For instance, those running their servers in virtualization environments can use their "snapshoting" ability, others might use live block-level replication to a failover server (drbd), or incremental backups to a centralized server allowing NFS-based emergency booting, or many other options I can't even imagine.

Therefore, each provider should choose and implement the one that best fits their needs.