set udp SocketOptionName.ReuseAddress to false, to not allow two regions

to bind to same port, as seems to be possible at least with mono 3.2.8,
 same as patch just pushed to core by justin. This is not necessary on
 windows, possible a bug on some mono versions.

OpenSim/Region/ClientStack/Linden/UDP/OpenSimUDPBase.cs

@@ -152,6 +152,8 @@ namespace OpenMetaverse
                     SocketType.Dgram,
                     ProtocolType.Udp);
 
+                m_udpSocket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, false);
+
                 try
                 {
                     // This udp socket flag is not supported under mono,