Add debug mode / command

[tjguk]

Trying to work out why some particular combination of machines / network is failing is difficult. At least it would be useful to output certain information: all interfaces / networks / default / gateways etc. (Basically anything we can come up with which might help).

I'm thinking of creating a main so the package can be run as python -mnetworkzero and then output whatever is useful.

[tjguk]

Or, perhaps better, add an "info" module which can be run via python -m but which can also be imported.