du: preserve suffix-only block size units

[wtcpython]

Fixes #13605.

Preserve unit suffixes for block sizes specified without a numeric prefix, including values from DU_BLOCK_SIZE, BLOCK_SIZE, and BLOCKSIZE. Numeric block sizes keep their existing output format.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks
⏩ 410 skipped benchmarks¹

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_{Comparing wtcpython:du-block-size-suffix (84179b1) with main (c9077c7)}

Footnotes

1. 410 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]

Binary size comparison:

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

Total size of compared binaries: 152.18 MB (+1004 KB, +0.65%)

Significant per-binary changes:
  comm     1.12 MB ->    2.33 MB  (+1.21 MB, +107.29%)

[wtcpython]

Would you mind taking a look when convenient? Thanks!

[Copilot]

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[wtcpython]

@sylvestre ready