numfmt: add --debug flag to print warnings about invalid input

[ChrisDryden]

The GNU numfmt utility has a debug flag that has a few integration tests in numfmt. It has debug messages for three things, when grouping is used in a locale that has no effect, no conversion option specified and "header ignored with command line input"

The grouping functionality is not yet implemented so I could only start with the debug messaging for the conversion option specified and the ignored with command line input debug message.

This makes 2 more tests out of the 72 remaining numfmt related tests pass.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing ChrisDryden:numfmt-debug-flag (eb508e8) with main (6d1df56)}

Summary

✅ 142 untouched benchmarks
⏩ 37 skipped benchmarks¹

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. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/shuf/shuf-reservoir (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/sort/sort-stale-thread-mem (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.