nice: fix Windows priority class and exit status

[Smicry]

Windows reports scheduling priority as one of six classes rather than a
nice value, so nice could not report the current niceness, and the
table it used to pick a class was not Cygwin's. Read the class the
current process runs at and map it with Cygwin's table in both
directions, and wait for the spawned command so that nice exits with
its status the way it does on Unix.

Closes #14484

[xtqqczze]

@Smicry please squash to clean the branch commit history

[codspeed]

Merging this PR will regress 1 benchmark

⚡ 2 improved benchmarks
❌ 1 regressed benchmark
✅ 390 untouched benchmarks
⏩ 54 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	five_38_bit_primes	1.7 s	1.9 s	-6.9%
⚡	Simulation	three_39_bit_primes	1,098 ms	838.2 ms	+31%
⚡	Simulation	thirteen_39_bit_primes	9.6 s	8.8 s	+8.41%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing Smicry:fix/nice-windows-priority-class (ee97c1a) with main (acd3d5b)²}

Footnotes

1. 54 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. ↩

2. No successful run was found on main (7e94315) during the generation of this report, so acd3d5b was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[Smicry]

@Smicry please squash to clean the branch commit history

done

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/id/setgid is no longer failing!