Chroot exec does not use bind-mounts for XDG dirs

[Larry Price]

Discovered on ask: https://askubuntu.com/questions/831830/libertine-terminal-applications-how-to-access-to-the-real-home-dir

When launching applications using libertine-launch, we use the `launch_application` method which does all the bind-mounting we need for us. But when using `exec`, we don't do any bind-mounting. `exec` is somewhat of a "hidden" command, but it would be appropriate for it to behave similarly to launch.

[Christopher Townsend (townsend)]

Hmm, exec is really intended to do "root" type operations inside a container, so bind-mounting directories from a user's home doesn't really make sense and it was intentional not to do so.

If a user needs to do command line operations in a chroot, they should install a terminal application in the container and use that.

[Larry Price (larryprice)]

After discussing this over IRC, we have decided that `exec` should only be for root-type operations. As such, we will actually be undoing the home directory bind-mounting which currently happens during an lxc `exec` to make this consistent across container types.