[MBR compatibility warning]

>Right, but, say, what _disk_ is it warning about?
>
>(yeah, i know, it's hard to provide that information, but without that
>information the message is _really_ annoying.)

Oh, foo. The device major and minor are available trivially; so I
guess you're saying you want the actual device name.

Hm. We could solve that generally by creating new bdevsw (and cdevsw?)
entrypoints
   	const  struct bdevsw  * (d_device)

and let the callee fish out the dv_xname.  Would you buy that, from an
abstraction viewpoint?

Or should we just change the readdisklabel/writedisklabel() signature
and have the caller pass dv_xname?