When we only have stdout and stderr it becomes multiplexed

This means we have an 8 byte header that we have to care about.

I'm not sure if it's just when we don't have stdin, or if it's when we have
stdout and stderr. (i.e. I don't know the behaviour if only stdout or only
stderr is attached)
Merge remote-tracking branch 'delfick/multiplexed' into non-interactive

Conflicts:
	dockerpty/pty.py