sort: deduplicate file descriptors in merge mode

[nonontb]

What This Does

This PR makes sort -m (merge mode) use less (minimum?) opened files.

The Problem

Before:

If you ran sort -m file.txt file.txt file.txt, the program opened file.txt three times eagerly — once for every time it appeared on the command line.
With lots of duplicates or a tight system limit on open files, this could fail.

If you tried to merge a file that was also your output file, the program had to create a temporary copy behind the scenes, using one more file.

GNU version has no issue running the test in #5714

The Fix

Now the program opens each unique file only once and Lazily and use Mmap (memmap2 - unsafe) to manage one FD for all input file duplicates including re-use of output file as inputs.

Result

Fix #5714

[github-actions]

GNU testsuite comparison:

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)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cp/sparse-to-pipe is no longer failing!
Congrats! The gnu test tests/tail/pipe-f2 is no longer failing!
Congrats! The gnu test tests/basenc/bounded-memory is now passing!
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)

[codspeed-hq]

Merging this PR will not alter performance

✅ 309 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing nonontb:feature/sort-input-fd-optimization (44259aa) with main (33b9156)}

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. ↩

[sylvestre]

please generate the files on the fly

[nonontb]

Done