Temporary backup name

[SimonHeimberg]

advantages of temporary backup name:

• script knows easy which backups are complete => no danger to try to resume a completed backup => lock can be set early without problems
• user sees easy which backups are complete

setting the lock early prevents concurrent backups more reliable

[SimonHeimberg]

I currently have some backups which are empty. Reason to find. If the backup name would be temporarely, I would have recognized the problem easier.