Make TNT respect on_blast, implement on_blast for some nodes

Implemented nodes:
- Steel Door: Ignores explosion
- Locked Chest: Ignores explosion
- Fire: Ignores explosion
- TNT: Starts burning
- Burning TNT: Explodes immediately
- Gunpowder: Starts burning
- Burning Gunpowder: Ignores explosion

mods/fire/init.lua

@@ -24,6 +24,9 @@ minetest.register_node("fire:basic_flame", {
 	on_destruct = function(pos)
 		minetest.after(0, fire.on_flame_remove_at, pos)
 	end,
+
+	-- unaffected by explosions
+	on_blast = function() end,
 })
 
 fire.D = 6