Support XKISS TNCs and enhance stack to use it

[ke6jjj]

Summary

XKISS is a protocol enhancement to TNCs which provides, among other things, notifications when packets have actually been transmitted over the air. This notification feature can be useful for improving the throughput and behavior of the station on the network.

This is a placeholder task for such a feature.

[ke6jjj]

As is typical of the Internet, there doesn't seem to be a reliable, centralized standard document for XKISS. We may and up having to read the source code for the BPQ BBS suite to fully figure it out.

[erikarn]

After digging into linbpq, it looks like John used 0xc as the command byte for ACKMODE.

tx[0] = 0x0c (for TNC 0): send following frame with ACK mode
tx[1], tx[2] = 16 bit ACK value
tx[3] = start of normal payload

RX is the same: rx[0] = 0x0c (plus the TNC 0), rx[1] and rx[2] are the tx'ed frame ACK value. it looks like the TNC just sends that as a status notice.

John also gave me patches to direwolf so I can start experimenting.