fix(timeout, uucore): use sigwait and timers for waiting

[Alonely0]

Does away with the current waiting loop for great good.

I had to use the nix crate for signals (even if we're trying to move away from it) because rustix has no plans to support the sigset APIs. I also had to manually use libc for the timers because nix has not caught up to redox, although at least it's very self-contained trivially safe.

Fixes: #9099, #11615 (and other duplicates, if any). All other open timeout issues should be re-assessed since it's a big backend change.

Opened as draft until I nail down platform support and an intermittent GNU test suite failure.

[codspeed-hq]

Merging this PR will not alter performance

✅ 327 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing Alonely0:fix-timeout-latency (968dc25) with main (bc3a413)}

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. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/dd/no-allocate is now passing!