fix: eliminate TOCTOU races in ln and tac by deferring is_dir() checks

[abendrothj]

ln: call hard_link() first, check is_dir() only on failure for diagnostics
tac: remove is_dir()/metadata() pre-checks, open file directly and let OS report errors (EISDIR, ENOENT)

Matches GNU coreutils behavior: operate first, diagnose after.

"it's better to ask for forgiveness, rather than for permission"

Closes #9450

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/io-errors. tests/misc/io-errors is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/dd/no-allocate is now passing!
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[codspeed-hq]

Merging this PR will not alter performance

✅ 288 untouched benchmarks
⏩ 42 skipped benchmarks¹

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_{Comparing abendrothj:fix/is_dir_TOCTOU_and_err_msgs (e8b98bc) with main (3883cea)}

Footnotes

1. 42 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:

GNU test failed: tests/date/date-locale-hour. tests/date/date-locale-hour is passing on 'main'. Maybe you have to rebase?
Congrats! The gnu test tests/dd/no-allocate is now passing!

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/date/date-locale-hour. tests/date/date-locale-hour is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/tail/pipe-f2. tests/tail/pipe-f2 is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/dd/no-allocate is now passing!
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/dd/no-allocate is now passing!

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[abendrothj]

It looks like codspeed had an issue installing in CI in the latest run, however everything else passes.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/date/resolution. tests/date/resolution is passing on 'main'. Maybe you have to rebase?
Note: The gnu test tests/unexpand/bounded-memory is now being skipped but was previously passing.

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[abendrothj]

^ CI failure seems unrelated or may just need a rerun, nothing related to timeout was modified in this PR.