Communicate SIGINT to the parent shell

[UnkindPartition]

This is the current work I have for #309. Please review and test it well before merging; I don't remember (and can't test right now) if it actually worked.

[santiweight]

Thank you for this! I will try to test this out on Monday at work. If I don't respond in a couple days and you still want review, please ping :)

[VictorCMiraldo]

I'd love to write a test for this, but my attempts behave the same at master and on this branch. I've tried:

• Write a testGroup that launches processes, then manually send SIGINT: I see the entire process killed and get my shell back in both branches
• Write a testGroup with one test that sends SIGINT to $$, only that test fails on both branches