Implement a devtype command.

List the device's protocol. The returned value is one of the following:
ata direct attach ATA or SATA device
satl a SATA device attached via SAS
scsi A parallel SCSI or SAS
nvme A direct attached NVMe device
mmcsd A MMC or SD attached device

Reviewed by: scottl@, rpokala@
Differential Revision: https://reviews.freebsd.org/D20950