Add .luacheckrc and do some code fixes

nether/.luacheckrc

@@ -0,0 +1,15 @@
+read_globals = {
+	-- Defined by Minetest
+	"minetest", "vector", "PseudoRandom", "VoxelArea", "ItemStack", "dump",
+	"string",
+
+	-- Mods
+	"default", "stairs", "creative"
+}
+globals = {"nether"}
+ignore = {
+	"212",
+		-- Unused argument
+	"411", "421", "422", "423", "431", "432",
+		-- Shadowing
+}