rename disc_scsi.c -> scsi.c

We don't have to prepend every file with "disc",
even if they implement some type of disc reading.
move scsi function declarations in scsi header

Also separate scsi_cmd and scsi_cmd_unportable,
although there is only unportable code right now.

scsi_cmd_unportable not mandatory to implement for every platform,
so it is in this header and not discid_private.h
However, it is recommended to implement.
add HACKING file