cat: improve throughput

[oech3]

$ truncate -s 1PB /tmp/huge
$ strace -o /dev/null -e inject=splice:error=ENOSYS cat /tmp/huge | pv>/dev/null
[ 547MiB/s]
$ strace -o /dev/null -e inject=splice:error=ENOSYS target/release/cat /tmp/huge | pv>/dev/null
[1.82GiB/s]

closes #11801

Note that CodSpeed does not catch it as splice code-path is used.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/printf/printf-surprise is now passing!

[oech3]

dirty code