umount-chroot: Call umount one mount point at a time

umount, called with multiple parameters, may return 0 even if many
mount points cannot be unmounted, due to an overflow in its exit
code.

We fix this by passing `-L 1` to `xargs`.

Finally, we sort mount points in inverse alphabetic order, to
minimize the number of loops required.
unmount-chroot: -n 1 is good enough
Merge pull request #876 from drinkcat/fix-unmount

umount-chroot: Call umount one mount point at a time