Add strict module

Also fix leaking globals found by it.

builtin/game/auth.lua

@@ -7,7 +7,7 @@
 function core.string_to_privs(str, delim)
 	assert(type(str) == "string")
 	delim = delim or ','
-	privs = {}
+	local privs = {}
 	for _, priv in pairs(string.split(str, delim)) do
 		privs[priv:trim()] = true
 	end
@@ -17,7 +17,7 @@ end
 function core.privs_to_string(privs, delim)
 	assert(type(privs) == "table")
 	delim = delim or ','
-	list = {}
+	local list = {}
 	for priv, bool in pairs(privs) do
 		if bool then
 			table.insert(list, priv)

builtin/game/item_entity.lua

@@ -56,7 +56,7 @@ core.register_entity(":__builtin:item", {
 			item_texture = core.registered_items[itemname].inventory_image
 			item_type = core.registered_items[itemname].type
 		end
-		prop = {
+		local prop = {
 			is_visible = true,
 			visual = "wielditem",
 			textures = {itemname},