luacheck, remove old compatibility stuff

.luacheckrc

@@ -0,0 +1,25 @@
+unused_args = false
+
+ignore = {
+    "431", -- Shadowing an upvalue
+	"432", -- Shadowing an upvalue argument
+}
+
+read_globals = {
+    "minetest",
+    "string",
+    "table",
+    "vector",
+    "default"
+}
+
+globals = {
+    "boards",
+    "display_api",
+    "font_api",
+    "ontime_clocks",
+    "signs",
+    "signs_api",
+    "signs_road",
+    "steles"
+}