Added some more debug statements that are enablable by atapi_debug in the
boot monitor.
Added SCSI_SENSE command that asks the drive, with request sense, about
diagnostics for the previous error. This command is called and the results
printed when atapi_debug is enabled and atapi errors occur.
Debug output also prints actual commands sent to the drive.
Suddenly, Andy's problem has vanished. Don't know why, minor changes
(e.g. moving the atapi packet data from stack to static) shouldn't have
fixed this.. But with the debug info, we stand a better chance of
finding out what it was if it occurs again.