refactor: use SpliceState enum instead of bool in splice functions

[xtqqczze]

Replace the ambiguous io::Result<bool> return type in pipe::splice_unbounded and pipe::splice_unbounded_broker with io::Result<SpliceState>.

This makes splice outcomes self-describing instead of relying on inverted boolean semantics (true meaning fallback required).

@oech3 sorry for the conflicts with #12359 and #12364, but I think this will make these PRs easier to review.

[oech3]

I don' t think this is readable. Please don't introduce unnecessary confliction.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (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/date/date-locale-hour (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)
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.

[oech3]

cc: reviewers
Please don't merge this. This breaks many PRs actually related with production quality.

[oech3]

If you don't like bool and flush, I just embed the required fallback to a unified function and make others private.
No need to merge this.

cc reviewers PLEASE DON'T MERGE THIS.

[oech3]

By the way, I previously tried to refactor *sum which is really low-priority. As a result it deffered very important (also for me) fix for its interfaces.