uucore: splice fast-path for head -c & preliminary for dd, tail, split

[oech3]

Closes #11843 Closes #11620 Fixes #11777
This should help achieving same perf with

$ cat /tmp/huge|pv>/dev/null
^C.8GiB 0:00:01 [35.8GiB/s]

at 3 utils in the future.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.

[cakebaker]

Was reviewer's comment removed?

I removed the original comment as the selected region was incorrect.

[oech3]

Many CI down

[cakebaker]

Thanks!

[oech3]

$ truncate -s 1PB /tmp/huge
$ head -c 1PB /tmp/huge | pv>/dev/null
^C.4GiB 0:00:01 [39.4GiB/s]
$ gnu9.11/head -c 1PB /tmp/huge | pv>/dev/null
^C49GiB 0:00:02 [2.26GiB/s]

I'm not sure how to utilize it at general scripting.
I'm currently doing run0 sh -c "cat *.iso > /dev/sdbN" since uutils/dd does not splice() yet.

[oech3]

(Also I'm waiting #11458)