Add a luacheckrc and fix errors reported by luacheck

* stairs is now added to depends.txt
* The mapgen perlin noise functions use a buffer now

.luacheckrc

@@ -0,0 +1,16 @@
+read_globals = {
+	"dump", "vector", "minetest",
+	"table", "math", "PseudoRandom", "VoxelArea",
+	"stairs", "stairsplus", "skins", "treecapitator",
+	default = {
+		fields = {
+			player_attached = {
+				read_only = false,
+				other_fields = true
+			}
+		},
+		other_fields = true
+	}
+}
+globals = {"snow"}
+-- ignore = {"421", "423"}