pr: stream simple layouts for non-file inputs

[mattsu2020]

Summary

This PR adds a streaming path for simple pr layouts so inputs such as
character devices and FIFOs can be processed incrementally instead of buffering
the entire input in memory.

The main goal is to avoid OOM-like behavior for inputs such as /dev/zero,
where pr can produce output line by line without waiting for a full read.

What changed

• add a streaming implementation for simple pr layouts
• use the streaming path for supported non-regular-file inputs
• align streaming output with the existing buffered behavior, including page trailer formatting
• fall back to the buffered path when the selected page layout leaves no printable content lines
• add regression tests for:
  • character-device input
  • FIFO/file parity for layouts that actually exercise the streaming path
  • FIFO/file parity in zero-content-line fallback cases

Related

• pr /dev/zero [SIGPIPE|SIGKILL] #11139

[sylvestre]

the list of commits isn't ideal, could you please improve it? thanks

[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)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!
Congrats! The gnu test tests/tail/pipe-f is now passing!

[codspeed-hq]

Merging this PR will not alter performance

✅ 339 untouched benchmarks
⏩ 46 skipped benchmarks¹

---

_{Comparing mattsu2020:pr_zero (339f865) with main (6e28633)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

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

[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/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cut/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/pipe-f is now passing!

[oech3]

Conflicting