get rid of most of the loop-devices code

get rid of most of the loop-devices code

-) loop devices are now attached in mountpoint_mount, and
immediately detached in order to set the auto-clear flag

Keeping track of loop-devices is otherweise next to
impossible and a security concern.
We mount the filesystems for the container. We do not
support full loop device access for containers for a simple
reason: once a container detached a loop device, the
startup of another container might reuse it, exposing its
devices to the first container, generating unwatned cross
container access permissions.

Loop devices are also set to auto-clear, so that we do not
need to worry about detaching them when stopping the
container.