added quota flag to mountpoints

added quota flag to mountpoints

quotactl(2) requires a path to the device node to work which
means we need to expose them to the container, luckily it
doesn't need r/w access to the device. Also, loop devices
will not detach from the images anymore with them being
still mounted in the monitor's mount namespace (which is
unshared from the host to prevent accidental unmounts via
lxc.monitor.unshare).

Note that quota manipulation currently does not work with
unprivileged containers.