Adverts broadcast over all networks

[tjguk]

The resolution to Issue #41 has the broadcast mechanism sending adverts to the global broadcast address and to the broadcast address of each network. This could result in, eg, an advert intended for local consumption (127.0.0.1) being broadcast to a remote machine which will then try to connect to it against its own localhost network.

Ideally the broadcast code should advertise an address only to the network which contains it (and the global broadcast address).