driver model: add struct device_driver::shutdown() method and device_present() helper.

->shutdown() is added to be explicitly called during reboot transitions by 
device_shutdown(). Drivers implementing this method should use it solely to quiesce
the device. Drivers do not have to free memory allocated by the driver, as it is 
implied that the system is rebooting; and, they must not fuss with the reference
counts. Devices are guaranteed not to disappear during the call, though their 
validity may be checked using device_present().

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