tail: improve throughput for -c +0 by splice()

[oech3]

Start supporting #11607 and #11785 .

tail's code path is very complex. So I do them with several PRs.
This PR covers

> truncate -s 9PB /tmp/huge
> target/debug/tail -c +0 /tmp/huge|pv>/dev/null
^C.4GiB 0:00:01 [39.4GiB/s]

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/retry. tests/tail/retry is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Note: The gnu test tests/tail/pipe-f is now being skipped but was previously passing.

[codspeed-hq]

Unable to generate the performance report

There was an internal error while processing the run's data. We're working on fixing the issue. Feel free to contact us on Discord or at support@codspeed.io if the issue persists.