IPV6CP: suggest an interface identifier option

Some peers seem to be sending no such option. The rfc says in that case
to try to send an option with a suggested value. Don't insist on it
however if the peer still doesn't send any.

That can fix IPv6 for some peers.

l2tpns.h

@@ -441,6 +441,9 @@ typedef struct
 	// last LCP Echo
 	time_t last_echo;
 
+	// Whether we tried to suggest the IPV6CP identifier option.
+	int tried_identifier;
+
 	// last unsolicited RA sent to user
 	time_t last_ra;