Icesheet biome: Add 'default:cave_ice' node to enable caves in land ice

Has 'is_ground_content = true'.
Drops 'default:ice'.

mods/default/mapgen.lua

@@ -820,7 +820,7 @@ function default.register_biomes(upper_limit)
 		depth_top = 1,
 		node_filler = "default:snowblock",
 		depth_filler = 3,
-		node_stone = "default:ice",
+		node_stone = "default:cave_ice",
 		node_water_top = "default:ice",
 		depth_water_top = 10,
 		--node_water = "",

mods/default/nodes.lua

@@ -65,6 +65,7 @@ default:snow
 default:snowblock
 
 default:ice
+default:cave_ice
 
 Trees
 -----
@@ -574,6 +575,7 @@ minetest.register_node("default:snowblock", {
 	end,
 })
 
+-- 'is ground content = false' to avoid tunnels in sea ice or ice rivers
 minetest.register_node("default:ice", {
 	description = "Ice",
 	tiles = {"default_ice.png"},
@@ -583,6 +585,16 @@ minetest.register_node("default:ice", {
 	sounds = default.node_sound_glass_defaults(),
 })
 
+-- Mapgen-placed ice with 'is ground content = true' to contain tunnels
+minetest.register_node("default:cave_ice", {
+	description = "Cave Ice",
+	tiles = {"default_ice.png"},
+	paramtype = "light",
+	groups = {cracky = 3, puts_out_fire = 1, cools_lava = 1, slippery = 3},
+	drop = "default:ice",
+	sounds = default.node_sound_glass_defaults(),
+})
+
 --
 -- Trees
 --