Dune doesn't kill process group when cleaning up

[rgrinberg]

Extracted from here. Dune should kill the entire process group. This requires killpg bindings.

[aalekseyev]

Copying my comment from there:

It's not simply a matter of writing bindings, it also means that we'd have to spawn every process in its own process group, which means that we need a patch for spawn and that Ctrl+C will no longer be sent to them etc. All of that is probably fine, just not such a self-contained change as one might expect.

[aalekseyev]

This is related to #3941