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

@@ -67,7 +67,7 @@ define Package/xl2tpd/install
 	$(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))