Implement a device based on the Linux SG 3 driver, e.g. for SCSI-to-USB bridge

[uweseimet]

A new device type SCSG shall use the Linux SG 3 (generic SCSI) driver. This device type directly exposes any device that the Linux SG driver supports as a SCSI device, usually mass storage devices.

As all commands are passed to the actual device instead of being emulated, INQUIRY does not report "SCSI2Pi:SCSI HD" etc., but the real name of the device, based on the device's INQUIRY data. The same applies for other device data like the SCSI level.

Notes:

• Linux limits the maximum SCSI transfer size per SCSI command. This means that s2p has to split large transfers into smaller chunks. s2p does this for block-oriented commands, but not for others.
• s2p can only support known SCSI commands, i.e. those listed on https://www.scsi2pi.net/en/scsi_commands.html. This is because the target (s2p) needs to know the number of bytes in each CDB. Unknown commands are rejected with INVALID COMMAND OPERATION CODE.
• Knowledge about each command is also required in order to correctly split large transfers into smaller ones, e.g. a single READ(10) transfer in several smaller READ(10) transfers with sizes within the Linux limits.

[uweseimet]

This feature has passed initial tests with an Atari TT, a USB stick, a USB floppy drive and a USB DVD drive.