ARCnet "cap mode" packet encapsulation. Used to get the hardware
acknowledge back to userspace. After the initial protocol byte every
packet is stuffed with an extra 4 byte "cookie" which doesn't
actually appear on the network. After transmit the driver will send
back a packet with protocol byte 0 containing the status of the
transmission:
0=no hardware acknowledge
1=excessive nak
2=transmission accepted by the receiver hardware

Received packets are also stuffed with the extra 4 bytes but it will
be random data.

Cap only listens to protocol 1-8.