Link up/down event handling

[thomastaranowski]

I'd like to handle physical link up/down events so I can monitor network state. It looks like I can stick a callback handler into ethernet.cpp : ProcessDuplexStatus() to do this, but I was thinking there was already a supported method, but darned if I could find it. Does anyone know if there's already a method for handling this? If not I'll post a patch on here, cause it seems generally useful.

Thanks!