Fixing issues with users that have no active pid

[ChrisDryden]

There's been an open issue for a while where a bunch of tests have been disabled in users and who because when going through the file lists of users it will return users that do not have an active PID. This issue is documented here and outlines that two things need to be fixed, first using the kill command to tell if a user is active and then adding an entry into the CICD yml so that the tests can run: #3219

This causing issues with the code coverage is documented here: #9060

The main functionality changes are that it now runs kill for the users with the 0 flag to be able to determine whether the user is active. To be able to run this without having to use unsafe I have included the nix library. Other than that the changes are to add the user fix in the yml thats decribed in the issue 3219 and re-enabled the tests.

A follow up would be to make the tests across the suite follow the new macro conventions. Theres a discrepancy in the one test that was disabled in 2022 that checks the version of GNU, the current tests ignore this and just flag a warning but this one was not updated to use the new macro.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)

[codspeed-hq]

CodSpeed Performance Report

Merging #9442 will not alter performance

_{Comparing ChrisDryden:who_check_pid (91b1724) with main (b8edd13)}

Summary

✅ 126 untouched
⏩ 6 skipped¹

Footnotes

1. 6 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)

[jtracey]

Looks good, all my comments are minor.

Kill is a pretty safe function (from a Rust safety guarantee perspective—it just takes two integers, no pointers or raw indexes), so if the nix dependency ends up causing problems in uucore, I'd say just use the unsafe libc calls. Otherwise, we already use nix in a bunch of utils, so if it works, it works.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)