control: Queue packets received Out-of-Order

Retransmissions are done very slowly, so we'd really better store the
Out-of-Order messages so that we can catch up quickly once we get the
missing piece.

l2tpns.h

@@ -275,6 +275,7 @@ typedef struct controls		// control message
 {
 	struct controls *next;	// next in queue
 	uint16_t length;	// length
+	uint16_t ns;		// sequence number
 	uint8_t buf[MAXCONTROL];
 }
 controlt;
@@ -506,6 +507,7 @@ tunnelt;
 
 typedef struct
 {
+	controlt *controlr;	// queue of OoO-received messages
 	int l2tp_fd;		// kernel acceleration UDP socket
 }
 tunnellocalt;