In the spirit of devices and buses, change driver refcounting model to 
match the way that devices and buses are done.

struct device_driver gets a ->present field, which is set on registration 
and cleared in driver_unregister(). get_device() checks the state of this
flag and returns NULL if cleared. 

Note that the horribly wrong remove_driver() is deprecated and simply BUG()s
when called. Please convert callers to use driver_unregister(). Updates to 
callers will be coming soon.

Note also that this still doesn't fix the race in which a driver module can
be removed while the refcount on a driver > 1. Near future work should help
to remedy it, but no solutions are guaranteed..