Enable "accept_rtadvd" on interfaces running rtadvd.

Without this, rtadvd runs but never advertises a default (IPv6) route.

MFC after:	1 week

etc/rc.d/rtadvd

@@ -36,17 +36,24 @@ rtadvd_precmd()
 	#
 	case ${rtadvd_interfaces} in
 	[Aa][Uu][Tt][Oo]|'')
+		command_args=
 		for i in `list_net_interfaces`; do
 			case $i in
 			lo0)	continue ;;
 			esac
 			if ipv6if $i; then
-				rtadvd_interfaces="${rtadvd_interfaces} ${i}"
+				command_args="${command_args} ${i}"
 			fi
 		done
 		;;
+	*)
+		command_args="${rtadvd_interfaces}"
+		;;
 	esac
-	command_args="${rtadvd_interfaces}"
+
+	for iface in ${command_args}; do
+		ifconfig ${iface} inet6 -accept_rtadv
+	done
 
 	# Enable Router Renumbering, unicast case
 	# (use correct src/dst addr)