everything "scsi_*", since we really are talking about the SCSI command
set, ATAPI transport not withstanding. Improve the names of many structures,
and prepend "SCSI_" onto all SCSI command opcodes. Place items described
by the SCSI Primary Commands document into scsi_spc.h.
(currently only CD-ROM drives on i386). The sys/dev/scsipi system provides 2
busses to which devices can attach (scsibus and atapibus). This needed to
change some include files and structure names in the low level scsi drivers.