du: show suffixes for unit-only block sizes

[mattsu2020]

du currently omits the unit label for block sizes such as KiB.
Preserve the label when the multiplier is omitted, while keeping explicit sizes such as 1KiB numeric-only, for both command-line options and environment variables.
Add regression tests for the display and option overrides.

Refs #7738.

[xtqqczze]

Code Quality / Style/lint (ubuntu-latest, all, true) (pull_request) failures tracked by #14916.

[codspeed]

Merging this PR will degrade performance by 3.21%

❌ 1 regressed benchmark
✅ 7 untouched benchmarks
⏩ 440 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	du_all_wide_tree[(5000, 500)]	36.9 ms	38.1 ms	-3.21%

Tip

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

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_{Comparing mattsu2020:codex/du-block-size-units (f2e7c2f) with main (2b85f6f)}

Footnotes

1. 440 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:

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