Trowel: Prevent hidden nodes from being dug.

As suggested by the player "felix7".

locale/techage.de.tr

@@ -385,6 +385,7 @@ Type=Typ
 Unlock=Entsperren
 Unlock connected chest@nif all slots are below 2000=Nachfolgende Kiste entsperren,@nsofern alle Speicherplätze <= 2000
 Update=Update
+Use a trowel to remove the node.=Verwende eine Kelle, um den Block zu entfernen.
 Usmium Nuggets=Usmium Nuggets
 Usmium Powder=Usmium Pulver
 WLAN Chip=WLAN Chip

locale/template.txt

@@ -383,6 +383,7 @@ Type=
 Unlock=
 Unlock connected chest@nif all slots are below 2000=
 Update=
+Use a trowel to remove the node.=
 Usmium Nuggets=
 Usmium Powder=
 WLAN Chip=

tools/trowel.lua

@@ -116,3 +116,35 @@ minetest.register_craft({
 		{"", "", "default:stick"},
 	},
 })
+
+local function get_new_can_dig(old_can_dig)
+	return function(pos, player, ...)
+		if M(pos):get_string("techage_hidden_nodename") ~= "" then
+			if player and player.get_player_name then
+				minetest.chat_send_player(player:get_player_name(), S("Use a trowel to remove the node."))
+			end
+			return false
+		end
+		if old_can_dig then
+			return old_can_dig(pos, player, ...)
+		else
+			return true
+		end
+	end
+end
+
+-- Change can_dig for already registered nodes.
+for _, ndef in pairs(minetest.registered_nodes) do
+	local old_can_dig = ndef.can_dig
+	minetest.override_item(ndef.name, {
+		can_dig = get_new_can_dig(old_can_dig)
+	})
+end
+
+-- Change can_dig for all nodes that are going to be registered in the future.
+local old_register_node = minetest.register_node
+minetest.register_node = function(name, def)
+	local old_can_dig = def.can_dig
+	def.can_dig = get_new_can_dig(old_can_dig)
+	return old_register_node(name, def)
+end