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.

[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)

[collinfunk]

FYI, I considered using splice for pipes in dd, but ultimately decided against it. My interpretation of the POSIX standard is that all reads, including seeks, must be performed in the specified blocksize. @pixelb WDYT?

[pixelb]

Yes dd is a low level tool, with operation specified by POSIX. You could have a splice option on iflag and oflag I suppose

[oech3]

How about block size = pipe size?

[oech3]

As an enduser, I'd like to make dd faster by default (at least for bs=64K~1M).
splice accepts length as its arg. So I think

must be performed in the specified blocksize.

is not a problem. I will try to use splice by default if it passes all existing GnuTests (and add missing test cases for posix requirement).