MFC r250298, r250301:
Fix byte order of ATA WWN when converting it to SCSI LUN ID.
MFC r251654, r251664:
Make CAM return and GEOM DISK pass through new GEOM::lunid attribute.

SPC-4 specification states that serial number may be property of device,
but not a specific logical unit.  People reported about FC storages using
serial number in that way, making it unusable for purposes of LUN multipath
detection.  SPC-4 states that designators associated with logical unit from
the VPD page 83h "Device Identification" should be used for that purpose.
Report first of them in the new attribute in such preference order: NAA,
EUI-64, T10 and SCSI name string.

While there, make GEOM DISK properly report GEOM::ident in XML output also
using d_getattr() method, if available.  This fixes serial numbers reporting
for SCSI disks in `geom disk list` output and confxml.
MFC r251661:
Replicate r242422 from ata(4) to mvs(4):
Only four specific ATA PIO commands transfer several sectors per DRQ block
(interrupt).  All other ATA PIO commands transfer one sector or 512 bytes
at one time.  Hardcode these exceptions in mvs(4).
This fixes timeout of READ LOG EXT command used by `smartctl -x /dev/adaX`.
Also it fixes timeout of DOWNLOAD_MICROCODE on `camcontrol fwdownload`.
MFC r251659:
Use CAM_DIR_NONE for requests without data.
Wrong values there confuse some drivers.