add overlayfs support

[radhermit]

This could do useful things by mapping files to overlay onto the chroot image. For example, there could be distro-specific plugins that could query the host systems pkgs and thereby overlay pkgs onto the chroot.

This could probably also help isolate multiple sessions running against the same chroot location at once. For example, now if a directory or file needs to be created in order to perform bind mounts for one chroot, another chroot using the same chroot path and running simultaneously will see the created directory or file (but obviously not the contents since its in a different mount namespace).