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/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/pid-pipe (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/rm/isatty (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.

[codspeed-hq]

Merging this PR will degrade performance by 3.03%

⚠️ 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
✅ 214 untouched benchmarks
⏩ 188 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	20.3 ms	-3.03%

Tip

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

---

_{Comparing wtcpython:du-block-size-suffix (b0f102f) with main (5633f03)}

Footnotes

1. 188 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: 148.97 MB (-132 KB, -0.09%)

Significant per-binary changes:
  [           1.10 MB ->    1.18 MB  (+76 KB, +6.74%)
  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%)
  mkfifo      1.09 MB ->    1.16 MB  (+72 KB, +6.45%)
  mknod       1.10 MB ->    1.17 MB  (+72 KB, +6.41%)
  chmod       1.15 MB ->    1.22 MB  (+68 KB, +5.76%)

[wtcpython]

Would you mind taking a look when convenient? Thanks!