ptx: handle -g by clap & reject 0

[oech3]

Fixes #13794

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/rm/isatty (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/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.

[cakebaker]

Thanks!