I've made a lot of changes to the wd driver recently.  It would be
good if people could test the current version and let me know if there
are any problems.  In particular:

* Do ST506 disks still work?

* Does logical block addressing always work?  (It will print `lba
addressing' in the attach messages if it's using logical block
addressing.)

* Does DMA work at all?  (I have no hardware I can test it on.)

I'd like to deal with IDE CD-ROMs, but I don't really want to spend
the time on it right now.  If anyone else is interested in working on
this, I can give you a quick summary of what needs to be done.