Don't overwrite listeners for different transports in config merging

Before this having listeners on same port for both tcp and udp would after
config merging step left only one of them.

Many thanks to Holger Weiß for noticing this.

src/ejabberd_config.erl

@@ -370,7 +370,10 @@ exit_or_halt(ExitText) ->
 
 get_config_option_key(Name, Val) ->
     if Name == listen ->
-            lists:keyfind(port, 1, Val);
+            case {lists:keyfind(port, 1, Val), lists:keyfind(transport, 1, Val)} of
+                {{_, Port}, false} -> {Port, tcp};
+                {{_, Port2}, {_, Trans}} -> {Port2, Trans}
+            end;
        is_tuple(Val) ->
             element(1, Val);
        true ->