Implement addChildProcess() to kill child processes on termination

* Implements the kill argument of OS.exec.
* Implements the automatic termination of processes spawned by OS.pipe.
* It notably kills the corresponding Tk instance when a VM terminates.

This still needs `{Application.exit 0}` or `{VM.kill {VM.current}}` to terminate of course as the Browser adds an IO node.
I guess it should be TerminateProcess then?