HEADS UP: Removing functions from driver API

[M. Warner Losh]

I'd like to remove from the driver API by making the following static:
	devclass_add_driver
	devclass_delete_driver
	devclass_find_driver

They aren't used, nor generally useful, by drivers in the current
tree.  The devclass_t routines are generally harder to lock than
necessary because they touch so much global data.  By eliminating
these from the API, its three fewer functions that need to be robustly
locked for external consumers.  Since they are basically unused today
anyway, I think it would be better to just reduce their scope.

Comments?

Warner