add port check for 8123

shadowsocks-csharp/Data/cn.txt

@@ -58,6 +58,7 @@ Please add at least one server=请添加至少一个服务器
 Server IP can not be blank=服务器 IP 不能为空
 Password can not be blank=密码不能为空
 Port out of range=端口超出范围
+Port can't be 8123=端口不能为 8123
 Shadowsocks {0} Update Found=Shadowsocks {0} 更新
 Click here to download=点击这里下载
 Shadowsocks is here=Shadowsocks 在这里

shadowsocks-csharp/Model/Configuration.cs

@@ -118,6 +118,10 @@ namespace Shadowsocks.Model
             {
                 throw new ArgumentException(I18N.GetString("Port out of range"));
             }
+            if (port == 8123)
+            {
+                throw new ArgumentException(I18N.GetString("Port can't be 8123"));
+            }
         }
 
         private static void CheckPassword(string password)