Revert "xl2tpd: rename netifd protocol handler to l2tpv2 to avoid clashes with l2tpv3"

This reverts commit 981f2da457.
OpenWrt wiki knew l2tp from l2tpv3tun, but luci knew l2tp from xl2tpd

net/xl2tpd/Makefile

 	$(INSTALL_DATA) ./files/options.xl2tpd $(1)/etc/ppp/
 
 	$(INSTALL_DIR) $(1)/lib/netifd/proto
-	$(INSTALL_BIN) ./files/l2tpv2.sh $(1)/lib/netifd/proto
+	$(INSTALL_BIN) ./files/l2tp.sh $(1)/lib/netifd/proto
 endef
 
 $(eval $(call BuildPackage,xl2tpd))

net/xl2tpd/files/l2tpv2.sh → net/xl2tpd/files/l2tp.sh

 	init_proto "$@"
 }
 
-proto_l2tpv2_init_config() {
+proto_l2tp_init_config() {
 	proto_config_add_string "username"
 	proto_config_add_string "password"
 	proto_config_add_string "keepalive"
 	no_device=1
 }
 
-proto_l2tpv2_setup() {
+proto_l2tp_setup() {
 	local config="$1"
 	local iface="$2"
 	local optfile="/tmp/l2tp/options.${config}"
 	xl2tpd-control connect l2tp-${config}
 }
 
-proto_l2tpv2_teardown() {
+proto_l2tp_teardown() {
 	local interface="$1"
 	local optfile="/tmp/l2tp/options.${interface}"
 
 }
 
 [ -n "$INCLUDE_ONLY" ] || {
-	add_protocol l2tpv2
+	add_protocol l2tp
 }