[ns] Multiple UDP traffic generator to one UDP source

[Giorgis Georgakoudis]

I builded a very simple app on top of the UDP agent of ns and the problem is 
that if I have two traffic generators both can send to the UDP agent of the 
traffic sink but when the sink is sending the ack (my application) the ack is 
send to the last node for which the "$ns connect $udpAgent1 $udpAgent2" has 
been called. How can I correct that?