fix(uucore): replace busy loop with pselect(2)

[Alonely0]

Fixes an issue in uu_timeout where there would always be 100ms latency. This PR supersedes #9123 due to platform support issues, and fixes #9099.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/wc/wc-proc. tests/wc/wc-proc is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[codspeed-hq]

CodSpeed Performance Report

Merging #9154 will not alter performance

_{Comparing Alonely0:timeout (18b80a9) with main (601564b)}

Summary

✅ 71 untouched
⏩ 57 skipped¹

Footnotes

1. 57 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:

GNU test failed: tests/df/unreadable. tests/df/unreadable is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/head/head-write-error. tests/head/head-write-error is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/ls/infloop. tests/ls/infloop is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/head/head-write-error. tests/head/head-write-error is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/df/total-verify is now being skipped but was previously passing.

[Alonely0]

Hey! Could someone with access to a macOS machine try to reproduce the spurious CI failures? I have been all morning (unsuccesfully) trying to get some useful logs or backtraces out of CI lol.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/df/unreadable. tests/df/unreadable is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/head/head-write-error. tests/head/head-write-error is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/ls/infloop. tests/ls/infloop is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/tail/tail-c. tests/tail/tail-c is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/pid. tests/tail/pid is passing on 'main'. Maybe you have to rebase?

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)