Hi Guix!

The attached patch allows showing more detailed status in parallel.
One of the reasons for doing that is for supporting another of my
patches, that allows specifying download tasks and build tasks numbers
separately, with a default of 1 for each (so 2 tasks in parallel by
default).

With verbosity level 2, all messages from all builds are shown
(interleaved, obviously), and messages for other levels are the same.

When only one build is performed, no changes are visible.

When multiple builds are happening in parallel, this patch shows one
line per running job (whether build or download), and messages above.
This will look like this:

module-import-compiled.drv  75% ▕█████████████████████████            ▏
openjdk-9.181-jdk  337.6MiB 4.3MiB/s 00:07 ▕█▌                ▏   8.8%


If there is no progress lines in the output of a build, it doesn't show
a spinner (yet), so no feedback that something is happening, but that's
planned for v2. One other issue is probably caused by having lots of
events (caused by build log output), that make status lines blink.
Another issue is when you ^C, the cursor is on the first status line.

My ideas for this patch are:

First, it's possible to go back a few lines with an ANSI escape code,
so basically print all status lines, then go back to the first line.

When an even needs to print a new line, let it do it from the first
status line, and print the status lines from below it.

Sometimes, an event will contain an incomplete line (for instance, it
ends with \r instead of \n), so I want to record the line to prevent it
from being overwritten by a status line. It's printed together with the
status lines, so we can go back to that line and print the rest of the
line when we get more of it.


Thoughts, ideas?