stdbuf: extract error message from OS & strip errno

[oech3]

No description provided.

[codspeed-hq]

Merging this PR will degrade performance by 3.49%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 1 regressed benchmark
✅ 33 untouched benchmarks
⏩ 369 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	du_wide_tree[(5000, 500)]	19.7 ms	20.4 ms	-3.49%

Tip

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

---

_{Comparing oech3:stdbuf-err (de535b5) with main (82e197a)²}

Footnotes

1. 369 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 (b7a7b40) during the generation of this report, so 82e197a was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 149.10 MB (+100 KB, +0.07%)

Significant per-binary changes:
  [           1.10 MB ->    1.18 MB  (+76 KB, +6.74%)
  mkfifo      1.09 MB ->    1.16 MB  (+76 KB, +6.81%)
  test        1.10 MB ->    1.18 MB  (+76 KB, +6.74%)
  install     1.25 MB ->    1.32 MB  (+72 KB, +5.62%)
  mkdir       1.10 MB ->    1.17 MB  (+72 KB, +6.38%)
  mknod       1.10 MB ->    1.17 MB  (+72 KB, +6.41%)
  chmod       1.15 MB ->    1.22 MB  (+68 KB, +5.76%)