du: start printing output immediately

[ceteece]

This PR updates du so that it continuously prints output as it is running, as opposed to waiting until all of the data has been collected before printing anything. A separate thread has been added which handles printing data (and errors) as they are produced by the main thread.

Fixes #5516

[ceteece]

This is not ready for review yet, just wanted to get the initial proof of concept out there. It definitely still needs a lot of refinement and cleaning up.

My current solution uses a separate thread for printing, but if that's overkill for this situation then I can switch things back to a single-threaded implementation (please let me know if you have any strong opinions on this!).

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/du/inacc-dest. tests/du/inacc-dest is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/tail/inotify-dir-recreate. tests/tail/inotify-dir-recreate is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/tail/truncate. tests/tail/truncate is passing on 'main'. Maybe you have to rebase?

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/inotify-dir-recreate. tests/tail/inotify-dir-recreate is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/tail/retry. tests/tail/retry is passing on 'main'. Maybe you have to rebase?

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/tail/symlink is no longer failing!
Congrats! The gnu test tests/tail/truncate is no longer failing!
GNU test failed: tests/du/inacc-dest. tests/du/inacc-dest is passing on 'main'. Maybe you have to rebase?

[ceteece]

GNU testsuite comparison:

Congrats! The gnu test tests/tail/symlink is no longer failing!
Congrats! The gnu test tests/tail/truncate is no longer failing!
GNU test failed: tests/du/inacc-dest. tests/du/inacc-dest is passing on 'main'. Maybe you have to rebase?

This test seems to be failing intermittently, likely due to some specific interleaving of prints from the main thread (which still prints some errors) and the printing thread. I haven't been able to reproduce the failure locally yet. Will dig into this more.

Edit:
Looks like this is the failing output:

FAIL: tests/du/inacc-dest
=========================

--- exp	2023-11-21 05:59:01.395501161 +0000
+++ out	2023-11-21 05:59:01.399501125 +0000
@@ -1,7 +1,7 @@
 .
-./a
 ./b
 ./c
 ./d
 ./e
-du: cannot read directory './c': Permission denied
+: cannot read directory './c': Permission denied
+du4	./a
FAIL tests/du/inacc-dest.sh (exit status: 1)

As I suspected, the error message (which is coming from the main thread) is being interleaved with the output from the printing thread. This should be pretty fixable.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/tail/truncate is no longer failing!
GNU test failed: tests/du/inacc-dest. tests/du/inacc-dest is passing on 'main'. Maybe you have to rebase?

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/tail/inotify-dir-recreate is no longer failing!

[cakebaker]

Good work, thanks :)