head: splice fast-path for -c

[oech3]

Closes #11491 . Needs cat from main for benchmark.

$ truncate -s 1PB huge
$ cat huge | target/debug/head -c 1P|pv>/dev/null
^C.5GiB 0:00:02 [38.7GiB/s]

$ target/release/head -c 200000000000 huge|pv>/dev/null
 186GiB 0:00:05 [37.2GiB/s]

[Edit] Fixes #11777

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/seq/seq-io-errors. tests/seq/seq-io-errors is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/rm/isatty (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/dd/no-allocate is now passing!

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/dd/no-allocate is now passing!

[oech3]

Summary
  head-c-splice -c 8192 /etc/pacman.conf ran
    1.03 ± 0.23 times faster than target/release/head -c 8192 /etc/pacman.conf

I was surprised that this is useful even for just 2 pages.