master: Add --backup to initialize in BACKINGUP

This applies only to an empty cluster and makes the cluster transition from the initial state RECOVERING to BACKINGUP without passing through state RUNNING:

(empty) -> RECOVERING -> VERIFYING -> STARTING_BACKUP -> BACKINGUP

This is the only way to reach BACKINGUP without having to pass through RUNNING first and manually transitioning to STARTING_BACKUP. It is the only way to reach state BACKINGUP fully automatically.