unexpand: accept a blank as a tab-list separator like GNU

[AlejandroCoronadoN]

GNU unexpand accepts both a comma and a blank between tab stops in -t/--tabs, so unexpand -t '2 3' behaves like unexpand -t '2,3'. uutils only splits on a comma, so the space form is rejected:

$ printf '  a   b\n' | unexpand -t '2 3' -a
unexpand: tab size contains invalid character(s): ' 3'

uutils' own expand already accepts both separators via an is_space_or_comma helper; only unexpand was inconsistent. This splits the tab list on a space or a comma too, mirroring expand. Verified byte for byte against GNU unexpand in the C locale for '2 3', ' 2 3', '2 3', '2, 3' and '1 4 7'; invalid values like -t x still error as before.

[codspeed-hq]

Merging this PR will not alter performance

✅ 290 untouched benchmarks
⏩ 117 skipped benchmarks¹

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_{Comparing AlejandroCoronadoN:unexpand-space-tab-separator (80248df) with main (3d09364)}

Footnotes

1. 117 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:

Skip an intermittent issue tests/rm/isatty (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/symlink (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)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Congrats! The gnu test tests/cp/link-heap is now passing!

[oech3]

Not a useful function. Please remove it and write code directly.

[AlejandroCoronadoN]

Good point, removed the helper and inlined it as s.split([' ', ',']). Thanks for the review!