Support for EN50606 (Unicable II) commands and replies.

Unicable I (EN50494) and Unicable II (EN50607) use the same physical bus for transmitting commands, with different parameters.

Unicable II has some commands that are bi-directional. These may have a reply of up to 5 bytes (0x74, followed by up to 4 bytes of data).
Can we consider extending dvb_diseqc_slave_reply.msg[] by one byte to accommodate this?

