Like the other objects, this allows a decent bit of cleanup. Details include:

- use rwsem in subsytem, instead of one in struct device_class.

- use refcount in struct kobject, instead of one in struct device_class.

- kill class's present flag. 

- kill class_list, since we can just use class_subsys's. 

- make interfaces instances of their class's subsystem. This allows us to 
kill struct device_class::intf_list, and struct device_interface::node.