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 ×23

⚡ 2 improved benchmarks
✅ 311 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Memory	tee_stdin_file[10000000]	102.8 KB	70.8 KB	+45.22%
⚡	Simulation	tee_stdin_file[10000000]	3,970.9 µs	169.2 µs	×23

---

_{Comparing oech3:tee-splice-m (d36fcc3) with main (2e04477)}

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:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)

[oech3]

I'm considering something like

curl-splice url://to/lage.iso | tee /dev/nvmeXnYm /dev/nvmeZnWm > saved.iso

if you feel this is a joke. Unfortunately, I don't know how to get curl-splice.

[oech3]

3,970.9 µs -> 169.2 µs ×23

Do not merge this before merging #12131 since this PR masks benchmark result for all unix targets.
Also saved RAM usage is fake (#11597). (But it is bounded and scaled by input size).