tail: warn when --pid=0 is ignored

[wtcpython]

Fixes #13108.

tail used PID 0 both as the default value and for an explicit --pid=0, so it could not emit GNU's ignored-PID warning when the option was supplied without following. Errors while writing that warning were also discarded.

Track whether --pid was explicitly supplied so PID 0 produces the warning without changing -f --pid=0 behavior. Propagate write failures for that warning and add regression coverage, including /dev/full on Linux.

[codspeed-hq]

Merging this PR will improve performance by 3.92%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 98 untouched benchmarks
⏩ 286 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	du_all_wide_tree[(5000, 500)]	16.9 ms	16.2 ms	+3.92%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing wtcpython:tail-pid-zero-warning (9355093) with main (b0a6096)}

Footnotes

1. 286 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/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/symlink (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)
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/unexpand/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/seq/seq-epipe is now passing!