Improve the shutdown process

[ivandavidov]

trafficstars

Right now MLL simply forces all processes to end and unmounts all partitions. However, if we have sparse overlay structure, the unmounting has to happen in specific order (unmount the sparse image before unmounting everything else) or otherwise MLL ends up with unmounting errors which may lead to data loss. At the moment these unmounting errors are just annoying but in the long run this should be improved.

One way to improve the shutdown process is to introduce another 'autorun' chain (see #133) which is invoked on reboot/shutdown. In this way MLL could easily manage not only proper unmounting, but also all kind of cleanup work which could be necessary for the proper behavior of the overlay bundles.