stop-debian: when killing all processed, first send TERM, then send KILL

This approach first asks all the processes to shut themselves down.  Then
anything left still running after 1 second is forced to quit.
fix detection of not fully mounted loopback image

Somethings when things go wrong when working with a loopback image,
everything is unmounted except for /data/debian.  That was falsely being
detected as a stopped state, it should show as started so that the user
can still try to stop it again.