Modesense6

These patches starts adding a basic ModeSense6 command. This command is incomplete and only
implements the ElementAddressAssignments page, but that is the only page we need in order to talk
to a media changer.

Later patches in the series adds initial support for READELEMENTSTATUS, so that we can query the device
for what storage and data transfer slots are in use.

Finally, add a tools/mtx.py example tool. This aims to be a mtx re-implementation but only supports
mtx.py -f /dev/changer status   so far