od: report actual file open errors

[wtcpython]

Fixes #12980.

od reduced file-open failures to three hard-coded messages, so errors such as opening a Unix socket were reported as I/O error instead of the platform's actual diagnostic.

Format file-open failures with uucore::error::strip_errno, preserving GNU-compatible errors without including raw OS error numbers. Add a Unix regression test that creates a socket and verifies the propagated diagnostic.

[codspeed-hq]

Merging this PR will not alter performance

✅ 345 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing wtcpython:od-preserve-open-error (2e8a373) with main (1d5f752)}

Footnotes

1. 46 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. ↩

[oech3]

Please extract those error from e too.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/dd/no-allocate is now passing!