Try to fix most warnings reported by luacheck

.luacheckrc

@@ -0,0 +1,46 @@
+-- Disable some (non critical) warnings
+-- unused = false
+unused_args = false
+redefined = false
+
+globals = {
+	"airutils",
+}
+
+read_globals = {
+	"DIR_DELIM",
+	"ItemStack",
+	"PseudoRandom",
+	"basic_machines",
+	"biomass",
+	"climate_api",
+	"core",
+	"creative",
+	"default",
+	"dump",
+	"emote",
+	"math",
+	"mcl_formspec",
+	"mcl_player",
+	"minetest",
+	"player_api",
+	"signs_lib",
+	"skins",
+	"string",
+	"technic",
+	"vector",
+	"wardrobe",
+}
+
+-- Per file options
+files["airutils_biofuel.lua"] = {
+	globals = {"basic_machines.grinder_recipes"},
+}
+
+files["lib_planes/utilities.lua"] = {
+	globals = {"player_api.player_attached.?", "mcl_player.player_attached.?"}
+}
+
+files["pilot_skin_manager.lua"] = {
+	globals = {"skins.skin_class.apply_skin_to_player"}
+}