[Mldonkey-users] More source propagation bugs

[Pierre Etchemaite]

Clients are identified by IP and port in an udp clients group. That leads to
two bugs:

* if your IP change (say your ISP automatically disconnects you every day),
you'll receive the whole udp clients group addresses again. That's a bit
inefficient.
* if you kill then restart your core, you usually keep the same IP, but
would have lost your udp clients group lists (they're not saved to disk
right now); An since everyone knows your address, no one will send you the
list again...