Exiting from shifter doesn't terminate forked processes

[scanon]

Imagine the following....

docker run -it --rm busybox

sleep 10000 &
exit
ps aux|grep sleep
..nothing there....

So, docker will kill all the processes in the container when the primary process ends. The same is not true of Shifter. It would probably be good if Shifter did the same as Docker here and cleaned things up.