existing code caused problems with some SCSI controllers.
A new sysctl kern.cam.ada.spindown_shutdown has been added that controls
whether or not to spin-down disks when shutting down.
Spinning down the disks unloads/parks the heads - this is
much better than removing power when the disk is still
spinning because otherwise an Emergency Unload occurs which may cause damage
to the actuator.
PR: kern/140752
Submitted by: olli
Reviewed by: arundel
Discussed with: mav
MFC after: 2 weeks
fd104c151b