Fix problem with merging same port, different ip listeners in config files

Also switch to function used by ejabberd_listener to extract that
information instead of rolling our own code for that.

src/ejabberd_config.erl

@@ -370,10 +370,8 @@ exit_or_halt(ExitText) ->
 
 get_config_option_key(Name, Val) ->
     if Name == listen ->
-            case {lists:keyfind(port, 1, Val), lists:keyfind(transport, 1, Val)} of
-                {{_, Port}, false} -> {Port, tcp};
-                {{_, Port2}, {_, Trans}} -> {Port2, Trans}
-            end;
+            [{Key, _, _}] = ejabberd_listener:validate_cfg([Val]),
+            Key;
        is_tuple(Val) ->
             element(1, Val);
        true ->