Almost TCP

[cceckman]

Implement "almost TCP", stream multiplexing over the serial connection.

• Stream multiplexing
• Handshaking
  • determine how many streams the the hardware device supports
  • Resets
  • Graceful shutdown
• Per-stream flow control

I've started prototyping this on the atcp branch.

ATCP explicitly should have similar semantics and flow to TCP; there's a chance that this could become a TCP implementation in the future.