Renamed dreamplug-detect to machine-detect: generalize.
Generalize machine-detect to support more than DreamPlugs.

machine-detect now returns 0 if it could identify the hardware and 1
if it can't.  The MACHINE environment variable contains the name of
the identified hardware.

It might be reasonable to add a VERSION environment variable later.
The network configuration file is now raspberrypi-ready.

Now we just need to figure out how to detect the raspberrypi hardware.