CAM Enhancements for AHCI driver work, import some structures from OpenBSD.

CAM Enhancements for AHCI driver work, import some structures from OpenBSD.

Add a CAM bit in the ccb to indicate that a polled operation is being
requested.  This is easier for the driver to find then putting it in
the periph structure.

Rename the scsi_read_capacity_data_long structure to
scsi_read_capacity_data_16 to match OpenBSD, and add a
command code for READ_CAPACITY_16 to match OpenBSD and reduce
confusion.  Also add missing fields to scsi_read_capacity_data_16.

Add SCSI_REV_SPC3.  Taken from OpenBSD.

Add a page description structure for UNIT_DEVID (page 0x83), taken from
OpenBSD.

Add unions for SCSI CDB command structures and response fields to improve
code readability.