Trigger on_rightclick regardless on the formspec meta field

Document behaviour for older clients.

doc/lua_api.txt

@@ -4317,9 +4317,11 @@ Definition tables
        ^ By default: Calls minetest.register_on_punchnode callbacks ]]
        on_rightclick = func(pos, node, clicker, itemstack, pointed_thing), --[[
        ^ default: nil
        ^ if defined, itemstack will hold clicker's wielded item
        ^ itemstack will hold clicker's wielded item
        ^ Shall return the leftover itemstack
        ^ Note: pointed_thing can be nil, if a mod calls this function ]]
        ^ Note: pointed_thing can be nil, if a mod calls this function
          This function does not get triggered by clients <=0.4.16 if the
          "formspec" node metadata field is set ]]

        on_dig = func(pos, node, digger), --[[
        ^ default: minetest.node_dig

src/game.cpp

@@ -3799,6 +3799,11 @@ void Game::handlePointingAtNode(const PointedThing &pointed,

		if (meta && meta->getString("formspec") != "" && !random_input
				&& !isKeyDown(KeyType::SNEAK)) {
			// Report right click to server
			if (nodedef_manager->get(map.getNodeNoEx(nodepos)).rightclickable) {
				client->interact(3, pointed);
			}

			infostream << "Launching custom inventory view" << std::endl;

			InventoryLocation inventoryloc;