sort: reject --batch-size arguments above the fd soft limit

[sylvestre]

sort only validated --batch-size against the file descriptor rlimit when the argument did not even fit in a usize. A value such as 20 with a soft limit of 19 was silently accepted, while GNU sort exits with status 2 and reports the maximum usable value.

Compare the parsed value with the soft limit minus the three descriptors that are always taken by stdin, stdout and stderr, and reuse the existing "too large"/"maximum --batch-size argument with current rlimit" messages for both the overflow and the out-of-range cases.

PR #11961 (deduplicating file descriptors during merge) targets a different part of the problem and does not touch this validation.

Should make test tests/sort/sort-merge-fdlimit.sh pass #11961
#13841

[Copilot]

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[github-actions]

GNU testsuite comparison:

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/sort/sort-merge-fdlimit is no longer failing!

[codspeed-hq]

Merging this PR will improve performance by 8.44%

⚡ 9 improved benchmarks
✅ 348 untouched benchmarks
⏩ 50 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	numfmt_stream_to_si	414.1 ms	291.6 ms	+42.01%
⚡	Simulation	numfmt_from_si[10000]	83.3 ms	75.7 ms	+10.09%
⚡	Simulation	numfmt_stream_to_si_precision	346.4 ms	330.1 ms	+4.93%
⚡	Simulation	numfmt_to_si[10000]	90 ms	86.4 ms	+4.11%
⚡	Simulation	numfmt_round_modes[("up", 10000)]	90.9 ms	87.3 ms	+4.06%
⚡	Simulation	numfmt_padding[(10000, 50)]	95.8 ms	92.1 ms	+4.01%
⚡	Simulation	numfmt_round_modes[("towards-zero", 10000)]	91 ms	87.5 ms	+3.97%
⚡	Simulation	numfmt_round_modes[("down", 10000)]	91.3 ms	87.9 ms	+3.95%
⚡	Simulation	numfmt_large_numbers_si[10000]	96.7 ms	93.2 ms	+3.83%

Tip

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_{Comparing sylvestre:fix-gnu-sort-fdlimit (fe188b0) with main (b2a617e)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 (8fe6e9e) during the generation of this report, so b2a617e was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩