dirname: support -- terminator

[mknos]

• Standards document has an example where "--" option terminator is used.. $(dirname -- file) [1]
• dirname is not a bash builtin on my Linux system
• GNU and OpenBSD dirname support "--" so add it here too
• GNU version supports multiple path arguments, but that does not follow the standard (this version already raised an error for multiple arguments)
• test1: "perl dirname -- $(pwd)/A" --> single file argument after -- is valid
• test2: "perl dirname a b" --> too many args
• test3: "perl dirname --" --> not enough args
• test4: "perl dirname -x -- -A" --> bad option error

1. https://pubs.opengroup.org/onlinepubs/9699919799/utilities/dirname.html

[coveralls]

Pull Request Test Coverage Report for Build 10407089446

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.7%) to 73.069%

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Files with Coverage Reduction	New Missed Lines	%
bin/units	3	70.77%

Totals
Change from base Build 10350849541:	-0.7%
Covered Lines:	350
Relevant Lines:	479

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💛 - Coveralls

[briandfoy]

Since you've completely replaced the program, you should get credit in the metadata