Allow setups without "volume" sections (for simple setups, if no grouping by "volume" is desired). Implement by allowing absolute paths for "snapshot_dir" and "subvolume" sections, which will implicitely also allow to snapshot/backup the root filesystem / without the need of mounting subvolid=5.

Allow configurations like this:

snapshot_dir /btrbk_snapshots
subvolume    /
subvolume    /home