[cross_platform_cmd] pass signals to running process from parent

[haf]

As a dev,
running albacore interactively in my terminal,
I may press CTRL+C to send SIGINT to ruby;
Albacore should trap that signal,
And pass it along to currently running processes.

Manifestation

Process.wait2 is cancelled/throws. Trap is handled and thread/process terminated.

Current behaviour

Terminates the other process. SIGTERM (-9)