Closing a pipe should close the exec

[paulbellamy]

Reproduce by:

1. Run a container: docker run -ti alpine /bin/sh
2. Run an exec from scope
3. Run a long-running process: apk --update add htop && htop
4. Click the exec's close-button or close the window
5. in the container run ps aux | grep htop and observe that the previous cmd is still running.

Found in #650

[tomwilkie]

Good luck with that: moby/moby#9098