tail: add --debug flag to show follow implementation mode

[ChrisDryden]

The GNU implementation has a --debug flag that has the main purpose of showing which mode tail is using on the system. It is also used in the updated GNU tail tests to validate that inotify is supported instead of searching to see if the header file is included.

This should unblock us from running a few of the skipped tail tests when the gnu tag is updated.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/follow-name is no longer failing!

[codspeed-hq]

Merging this PR will improve performance by 3.64%

Summary

⚡ 1 improved benchmark
✅ 139 untouched benchmarks
⏩ 37 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	tsort_input_parsing_heavy[5000]	73.7 ms	71.2 ms	+3.64%

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_{Comparing ChrisDryden:tail-debug-flag (bda86ab) with main (20a5c3a)}

Footnotes

1. 37 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. ↩