nproc: Minor cleanup

[oech3]

• Use libc::sysconf at unix.
• Make funcs oneliner.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/rm/isatty is no longer failing!
Congrats! The gnu test tests/dd/no-allocate is now passing!
Congrats! The gnu test tests/tail/tail-n0f is now passing!

[codspeed-hq]

Merging this PR will degrade performance by 4.79%

❌ 1 regressed benchmark
✅ 301 untouched benchmarks
⏩ 44 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	shuf_input_range[1000000]	84.3 ms	88.5 ms	-4.79%

---

_{Comparing oech3:patch-2 (3c3eb94) with main (e9ad8b8)}

Footnotes

1. 44 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. ↩

[cakebaker]

Thanks!