add private mode (#897)

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Co-authored-by: Sijie.Sun <sunsijie@buaa.edu.cn>

easytier/src/common/config.rs

@@ -37,6 +37,7 @@ pub fn gen_default_flags() -> Flags {
         disable_kcp_input: false,
         disable_relay_kcp: true,
         accept_dns: false,
+        private_mode: false,
     }
 }