Add `--umount-outside` option

This helps to hide mount information outside chroot path from
/proc/mounts, etc.

However, umount is VERY SLOW and the feature is practically
unusable for some cases (ex. online judge system).

When in a hurry, try https://lkml.org/lkml/2014/9/29/554
Or just wait for kernel 3.18.

Help OCD