util/OptionParser: collect remaining arguments

Allow the caller to use a simple "for" loop without checking
arguments.

src/util/OptionParser.cxx

@@ -53,7 +53,9 @@ OptionParser::ParseNext() noexcept
 		option_raw = arg;
 		return true;
 	}
+
 	option = nullptr;
 	option_raw = nullptr;
+	*remaining_tail++ = arg;
 	return false;
 }