Add legacy +POS/-POS handling in sort to pass GNU sort-field-limit test

[karanabe]

This change translates GNU’s legacy +POS1 [-POS2] sort key syntax into -k form before clap parsing, gating it by _POSIX2_VERSION to match GNU behavior, and adds by-util tests to ensure acceptance/rejection behavior matches GNU and that the sort-field-limit compatibility test passes.

#9127

[github-actions[bot]]

GNU testsuite comparison:

Congrats! The gnu test tests/sort/sort-field-limit is no longer failing!

[github-actions[bot]]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/sort/sort-field-limit is no longer failing!

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #9501 will degrade performances by 4.51%

_{Comparing karanabe:test/fix-sort-field-limit (a10c06e) with main (64203e3)}

Summary

⚡ 1 improvement
❌ 2 regressions
✅ 124 untouched
⏩ 6 skipped[^skipped]

:warning: Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	sort_ascii_c_locale	21.6 ms	22.6 ms	-4.51%
⚡	sort_numeric	24 ms	23.5 ms	+2.04%
❌	tsort_input_parsing_heavy[5000]	82.1 ms	83.8 ms	-2.04%
[^skipped]: 6 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[bot]]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/sort/sort-field-limit is no longer failing!

[github-actions[bot]]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/sort/sort-field-limit is no longer failing!

[karanabe]

@cakebaker Thanks for the review! I've addressed all the comments. Regarding the test failure, test_tail::test_retry6 stdout does not seem to be related to this change and appears to be failing elsewhere as well. Could you please take another look?

[cakebaker]

Congrats! The gnu test tests/sort/sort-field-limit is no longer failing!

Kudos, thanks!