util.argparse: Revise 553c6204fe5b with a different approach

The second return value is (not insensibly) assumed to be an error. Instead of
returning a value there in the success case, copy the positional arguments
into the existing opts table.

util/argparse.lua

@@ -5,7 +5,7 @@ local function parse(arg, config)
 	local parsed_opts = {};
 
 	if #arg == 0 then
-		return parsed_opts, arg;
+		return parsed_opts;
 	end
 	while true do
 		local raw_param = arg[1];
@@ -47,7 +47,10 @@ local function parse(arg, config)
 		end
 		parsed_opts[param_k] = param_v;
 	end
-	return parsed_opts, arg;
+	for i = 1, #arg do
+		parsed_opts[i] = arg[i];
+	end
+	return parsed_opts;
 end
 
 return {