The screen real-estate is already reserved, the values are dumped and
restored by the on-DSP code, why not make something out of these values ?
Allows following:
- where exactly send_back was called from ($st1)
- the boundaries and progress of the innermost BLOOP{,I} ($st0, 2 and 3)
  up to send_back's call

The current code expects new mail almost immediately after the last map was sent for it to be saved properly. However, I have a test program that ends up looping for 32768 iterations before it sends more mail; this resulted in an incomplete result dump. I've changed it to wait a frame between checking for mail, which solves that issue. This does slow down dumping, but the end speed matches the speed at which the UI updates the registers so this isn't a big deal (the UI waits a frame between mail normally). (Theoretically, it could take even longer for dumping to finish, so this is not a perfect solution. However, for tests that take that long to run, it would be better to save the existing results instead of re-running the test and saving that; that'd be something to do with later improvements.)

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

also print summed $prod and individual $sr bits