fix(pipes): return io::Result

[xtqqczze]

Follow-up #12408

Previously, some public function signatures exposed rustix::io::Result. This does not match the intended abstraction boundary, since these functions are not documented as thin wrappers around rustix.

[oech3]

No. This is intended because we discard all of splice's error. Conversion is not required. I strongly disagree with this change.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout-group (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cut/cut-huge-range is now passing!