A Red Hat QA engineer hit in practice a race condition in dbus-uuidgen
where it could leave an empty file.

dbus-uuidgen (_dbus_create_uuid_file_exclusively) formerly created an
empty file in the path to the uuid, then filled it in.  At some point,
the internal libdbus _dbus_string_save_to_file became atomic on Unix
at least (doing the save to temp file, fsync(), rename() dance).

So _dbus_create_uuid_file_exclusively doesn't need to create the file
beforehand anymore.  However, it *does* need the file to be
world-readable, unlike all other consumers of
_dbus_string_save_to_file.  So add a "world_readable" argument.