enter-chroot: Unmount /sys/fs/selinux, remount fake directory
enter-chroot: Unmount /sys/fs/selinux, remount fake directory

Fixes #2655.

We first unmount /sys/fs/selinux, which fixes a number of issues
(but not all).

Then, we create a fake, read-only, /sys/fs/selinux tmpfs, with
its enforce flag set to 0. This is necessary for precise to be able
to add users.

dnschneid/crouton,dnschneid/crouton,dnschneid/crouton,dnschneid/crouton,dnschneid/crouton,dnschneid/crouton