Hotpluggable ETHInterface for Mac OSX

This patch is for people who use Apple OSX and a USB ethernet card, when you unplug the card, it vanishes unlike the thunderbolt connector.

To use this patch, disable {"chroot": "/var/run"} in the conf because cjdns needs access to /dev/bpf* whenever a device gets plugged in. Then activate it by changing the eth interface from all to * (literal splat).

Good luck and -report- fix bugs.