df: split platform specific code

[nikolalukovic]

might be worth it to take a look again at these PRs before this one gets merged:

• fix df m flag and missing suffix #12448
• Fix df output on Windows #12244
• df: add a fallback path for statfs with a mount table #12072

EDIT: this one too

• df: don't show entries previously considered best #12071

[codspeed-hq]

Merging this PR will degrade performance by 3.91%

⚠️ 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
✅ 177 untouched benchmarks
⏩ 229 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	du_summarize_balanced_tree[(5, 4, 10)]	16.1 ms	16.8 ms	-3.91%

Tip

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

---

_{Comparing nikolalukovic:df-platform-split (d6655d5) with main (a9b64fc)²}

Footnotes

1. 229 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 (71ebb92) during the generation of this report, so a9b64fc was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/dd/no-allocate is now passing!
Congrats! The gnu test tests/tail/tail-n0f is now passing!