``__attrs__init__()`` will now be injected if ``init=False``, or if ``auto_detect=True`` and a user-defined ``__init__()`` exists.

This enables users to do "pre-init" work in their ``__init__()`` (such as ``super().__init__()``).

``__init__()`` can then delegate constructor argument processing to ``self.__attrs_init__(*args, **kwargs)``.