introduce static mut IS_POSIXLY_CORRECT

[xtqqczze]

No description provided.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/io-errors (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/seq/seq-epipe is now passing!
Congrats! The gnu test tests/tail/pipe-f is now passing!
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[RenjiSann]

Why would you introduce so much unsafe here ? Is it motivated by a significant gain in performance ?

[codspeed-hq]

Merging this PR will not alter performance

✅ 367 untouched benchmarks
⏩ 50 skipped benchmarks¹

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_{Comparing xtqqczze:static-posixly-correct3 (339547e) with main (4bae306)²}

Footnotes

1. 50 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. ↩

2. No successful run was found on main (a5d6966) during the generation of this report, so 4bae306 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[xtqqczze]

These changes are not correct, as we have entry points other than uumain, such as uu_cp::copy.