Allows sending messages only to a group of users

In some circumstances (like proximity, missions...) we may want
to target messages to only a sub set of all the users connected
to the server. Adding this optional parameter to SendModPacketToAll
and SendChatMessageToAll allows that while being backwards
compatible with the API
Fixes some problems introduced in the non interactive mode
Changes the SetLocalTraffic parameter to make it more readable
so now SetLocalTraffic(true) means to ENABLE local traffic while
SetLocalTraffic(false) means DISABLE local traffic

Client/COOPAPI.cs

@@ -266,10 +266,10 @@ namespace CoopClient
         /// <summary>
         /// Enable or disable the local traffic for this player
         /// </summary>
-        /// <param name="stop"></param>
-        public static void SetLocalTraffic(bool stop)
+        /// <param name="enable"></param>
+        public static void SetLocalTraffic(bool enable)
         {
-            Main.DisableTraffic = stop;
+            Main.DisableTraffic = !enable;
         }
 
 #if DEBUG