xl2tpd: add new config option checkup_interval.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

net/xl2tpd/files/l2tp.sh

@@ -15,6 +15,7 @@ proto_l2tp_init_config() {
 	proto_config_add_string "pppd_options"
 	proto_config_add_boolean "ipv6"
 	proto_config_add_int "mtu"
+	proto_config_add_int "checkup_interval"
 	proto_config_add_string "server"
 	available=1
 	no_device=1