Refactoring: Replace shellquote() with printf %q

[inkarkat]

I didn't know about printf's capability when I introduced quoting 10 years ago. The %q format will do the quoting, and -v VAR can be used to reassign to the variable.

Note: The shellquote() function has been exported for reuse by add-ons. I don't think anyone has ever used that (it was mostly intended for my own, extensive extensions, and I never used it), and is trivial to move away from, anyway.

• [X] Fork the repository and create your branch from master.
• [ ] If you've added code that should be tested, add tests!
• [X] Ensure the test suite passes.
• [X] Format your code with ShellCheck.
• [X] Include a human-readable description of what the pull request is trying to accomplish.
• [X] Steps for the reviewer(s) on how they can manually QA the changes.
• [ ] Have a fixes #XX reference to the issue that this pull request fixes.