Detect interface / address arrival and departure #3

This is needed so dhcpsd can be used easily on macOS in place of the DHCP server in the UTMS framework.

dhcpsd needs to start before anything else.
Once running, the macOS DHCP server won't be started.
UTMS will create the needed interfaces based on VM status - when one is up the bridge100 interface is created to serve DHCP from and it will be removed when all VMs have stopped.