who: fix panic in -q/--count on stdout write error

[koopatroopa787]

Summary

who -q (--count) prints a user list and a count line using println!, which aborts (panics) if stdout write fails — e.g. when output is redirected to /dev/full:

$ who -q > /dev/full
thread 'main' panicked at library/std/src/io/stdio.rs:1165:9:
failed printing to stdout: No space left on device (os error 28)
Aborted (core dumped)

Fix: replace the two println! calls in the short_list branch of Who::exec() with writeln! on a locked stdout handle, so errors are propagated via ? and reported gracefully.

Fixes #13388

Changes

• src/uu/who/src/platform/unix.rs: replace println! with writeln!(out, …)? for user list and count output
• tests/by-util/test_who.rs: add test_count_write_error (Linux only, via /dev/full) to confirm the error is reported rather than causing an abort

[Copilot]

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[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/expand/bounded-memory is now passing!
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!

[codspeed-hq]

Merging this PR will not alter performance

✅ 339 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing koopatroopa787:fix-who-count-write-error (ea4ab22) with main (ef50d8b)}

Footnotes

1. 46 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. ↩