Ivanpauno/kill dangling subprocesses

[anton-matosov]

Fork of https://github.com/ros2/launch/pull/632, rebased on latest rolling branch. What changed: Fix all type issues and missing null checks

Original message

Fixes https://github.com/ros2/launch/issues/545.

I used psutil to figure out children of a process recursively. It's an easy way to handle this issue platform independently.

For posix OSs, we could send a signal to the process group, but for that we should create a new process group when launching a process, which I'm not sure if it's the best ideal.