When receiving unexpected UDP packets, check if they match a known IP and forward them.

This will not work for many clients that come from a single IP but will
work-around issues, when clients are behind a NAT that keeps their UDP port
state for shorter time than DPD.

src/ipc.proto

@@ -138,7 +138,7 @@ message tun_mtu_msg
 /* UDP_FD */
 message udp_fd_msg
 {
-
+	required bool hello = 1 [default = true]; /* is that a client hello? */
 }
 
 /* SESSION_INFO */