Patch from Julian. Commit message by me.

Patch from Julian.  Commit message by me.

cd.c:
Initialize channel info in CDIOCSETVOL ioctl.
Correct CDIOCSTOP and CDIOCEJCET ioctls to use scsi_stop_unit
instead of scsi_start_unit.
Add CDIOCALLOW and CDIOCPREVENT ioctls.

ch.h:
Return EBUSY instead of ENXIO if the device is already in use.

scsi_base.c:
Add scsi_stop_unit routine.

sd.c:
Add mising indirection through sc_link to sd_get_parms routine
when checking for media loaded.

st.c:
Return EBUSY instead of ENXIO if the device is already in use.
Clear the SDEV_WAITING flag in ststart if we do the wakeup call.