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

The problem here is that umount, called with multiple parameters,
may return 0 even if some mount points cannot be unmounted.

Looking at umount source, I suspect there is an overflow as error
codes are summed up (see
https://git.kernel.org/cgit/utils/util-linux/util-linux.git/tree/sys-utils/umount.c
).

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

Finally, we sort mount points by string length, so that we minimize
the number of loops required.