Switch from legacy ata(4) to CAM-based ATA

Hi.

With 9.0 release approaching quickly, I believe it the best time now to
manage migration from legacy ata(4) ATA to the new CAM-based one. New
ATA code present in the tree for more then a year now, used by many
people and proved it's superior functionality and reliability. The only
major issue with it now is the migration process. Sooner or later we
have to pass it, but due to major UI and API changes we can't do it
after 9.0 release. So I propose to do it the next Sunday (April 24) to
have as much time for troubleshooting as possible.

I have prepared the following patch to do it:
http://people.freebsd.org/~mav/ata_switch.patch

I haven't added geom_raid to the kernel configurations because we have
no other GEOM classes there. But tell me if you thing I should.

If somebody has any problems with new ATA stack, please repeat your
tests with latest HEAD code and contact me if problem is still there.
Next three weeks before BSDCan I am going to dedicate to fixing possibly
remaining issues.

-- 
Alexander Motin