tee: add splice() and tee() zero-copy fast-path

[oech3]

Closes #11466 .

> tee </tmp/huge |pv >/dev/null
^C58GiB 0:00:01 [2.58GiB/s] 
> tee-splice </tmp/huge |pv >/dev/null
^C.6GiB 0:00:02 [38.4GiB/s]

[codspeed-hq]

Merging this PR will improve performance by 6.48%

⚡ 1 improved benchmark
✅ 322 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	tee_stdin_file[10000000]	179.5 µs	168.6 µs	+6.48%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing oech3:tee-splice-m (4b807b0) with main (d01be5b)}

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. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/retry (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)
Note: The gnu test tests/dd/no-allocate is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!

[oech3]

@sylvestre ping

[oech3]

This PR is ready.