`ExecFuture::resolveKill` uses `proc_terminate`, both when we **really** want to kill
the inferior process (e.g. from the destructor), but also when we just want to time
out.
This is problematic when trying to impose a timeout on a process that itself spawns
children. Because `proc_terminate` doesn't kill children, and because SIGKILL doesn't
give the child process a chance to clean its children for us, I end up with a bunch of
stuff still running.