This value is not normally acted upon by the sg driver. It is provided
so the user can identify the request. This is useful when command queuing
is being used.
The "abnormal" case is when SG_SET_FORCE_PACK_ID is set and a 'pack_id'
other than -1 is given to read(). In this case the read() will wait
to fetch a request that matches this 'pack_id'. If this mode is used
be careful to set 'dxfer_direction' to a valid value (actually any of
the SG_DXFER_* values will do) on input to the read(), together with
the wanted pack_id.
The type of pack_id is int.