Add periodic RA sends

The RFC indeed say that we should send them periodically.  We were
previously only sending them along LCP echo replies, but echo requests
are typically sent only when there is no trafic, which RA need to be
sent even when there is trafic.

l2tpns.h

@@ -56,6 +56,8 @@
 #define ECHO_TIMEOUT	10		// Time between last packet sent and LCP ECHO generation
 #define IDLE_ECHO_TIMEOUT	240		// Time between last packet seen and session shutdown
 #define BUSY_WAIT_TIME	3000		// 5 minutes in 1/10th seconds to wait for radius to cleanup on shutdown
+#define AdvDefaultLifetime	9000
+#define RtrAdvInterval	1800
 
 #define MP_BEGIN        0x80            // This value is used when (b)egin bit is set in MP header
 #define MP_END          0x40            // This value is used when (e)nd bit is set in MP header
@@ -434,6 +436,9 @@ typedef struct
 	// last LCP Echo
 	time_t last_echo;
 
+	// last unsolicited RA sent to user
+	time_t last_ra;
+
 	// Last Multilink frame sequence number received
 	uint32_t last_seq;