ymodem-1k file transfer capabilities for Agon VDP

[envenomator]

This PR delivers ymodem file transfer capabilities to the Agon VDP:

• Full bi-bidirectional file transfers using the industry-standard ymodem-1K protocol
• Multi-file batched transfers in a single session
• Automatic sending of filename(s) as part of the protocol. No more typing in filenames at the receiving end
• Each sending side will wait for the receiver to start, as per the ymodem specs. So one can just start sending to the Agon first, start the ymodem client on the Agon later, and the session starts
• No impact to other services, like Hexload
• Makes use of the existing 23,28 sequence for hexload. If no additional byte follows immediately, Hexload starts as normal. 23,28,1 starts the ymodem protocol in receive mode, 23,28,2 starts the ymodem protocol in send mode
• Tested to work with industry-standard ymodem clients, such as lrzsz (sz/rz in --ymodem mode), python ymodem module, TeraTerm on Windows
• Created custom (faster and more simple) linux/mac cli ymodem clients (https://github.com/envenomator/agon-ymodem/releases), also the ymodem.bin client for the Agon

Been using this for a while now, simultaneously with hexload, and it's been solid.

[envenomator]

Found a bug when the a file larger than 255K blocks is received by the VDP, and the content of the 256th block begins with a zero, currently incorrectly interpreted as end-of-batch, while more data should be received. I'll write a bugfix in the next few days