timeout()/untimeout() API:
- Clients supply callout handle storage, thus eliminating problems of
resource allocation.
- Insertion and removal of callouts is constant time, important as
this facility is used quite a lot in the kernel.
The old timeout()/untimeout() API has been removed from the kernel.
Clean up the name space here a bit.
Add a `busname' element to struct ofprobe (now struct ofbus_attach_args), and
check against it.
Nuke `ofroot'.
Rename the `openfirm' attribute to `ofbus'.
