pipes:rs: replace unnecessary splice by read-ahead hint

[oech3]

• drop 2 pub
• dedup 2 fcntl()s and reuse it as a hint to bypass splice without broker
• read ahead for splice (1st splice is slower than 2nd splice since page caches are mising)
• remove 2 splices from splice_unbounded as the hints are enough

dropping cache by echo 1 | run0 tee /proc/sys/vm/drop_caches fails on hyperfine

$ time target/release/cat-nofadv /lib/chromium/chromium>/dev/null
Executed in  284.67 millis    fish           external
$ time target/release/cat-fadv /lib/chromium/chromium>/dev/null
Executed in  272.77 millis    fish           external

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/head/head-write-error is no longer failing!
Congrats! The gnu test tests/cut/bounded-memory is now passing!
Congrats! The gnu test tests/printf/printf-surprise is now passing!

[cakebaker]

Thanks!