Should have an option to terminate sandbox when PID 2 dies

I saw that if a flatpak app somehow triggers another process to be spawned, when it quits, that spawned process can linger around.
That's bad, because if another app runs later, it may find the initial process and pick it up instead of spawning it again, except that the old process may be pointing to paths or state that don't exist anymore.

Discussing this with @matthiasclasen on IRC, we thought that bubblewrap should provide an option to terminate the sandbox also when the PID 2 dies, to prevent this from happening.