Set networking's UNKNOWN_SETUP by default.

This allows the setup process to configure default networking (for 1
ethernet port) whether or not we can recognize the device.  It seems
like the single-ethernet port configuration is a reasonable default in
just about all cases.
Networking setup now based on number of wired interfaces.

Added interface_detect.py from:

    https://github.com/NickDaly/interface-detect

If we detect two wired interfaces, run the "DreamPlug" setup (eth0 and
eth1), whereas if we detect only one, run the "RaspberryPi" setup
(eth0).  Unfortunately, having only one wired interface makes the
network configuration much more difficult.  However, we now have
enough information to be able to make the correct configuration
choices.