Add possibility to "whitelist" modems used by network

[MoonlightOwl]

Sometimes you want to use only one of your modems (with specific address) in your local net.
Or two. Or linked card instead of "wi-fi" modem.

[Fingercomp]

Right now, for simplicity's sake, we use only one modem in the network to avoid cycles. The modem can be set using the zn.modem setting -- it's the proxy of the modem you want to use.

I'll consider adding the feature. However, this would make the code more complex.