Add percentages to totals

[Arrigi]

It might be interesting to add percentages of total raw space to the totals at the end, indicating storage efficiency. Similar to how you'd expect 50% with 2-disk RAID1 and 66% with RAID5 on 3 disks, users might be interested in knowing what their weird 11-disk btrfs 3-parity raidz3 frankenstein would do.