lint: Fix all ShellCheck errors and most warnings

[hyperupcall]

Before submitting a pull request, please make sure the following is done:

• Fork the repository and create your branch from master.
• If you've added code that should be tested, add tests!
• Ensure the test suite passes.
• Format your code with ShellCheck.
• Include a human-readable description of what the pull request is trying to accomplish.
• 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.

This improves the code quality of todo.sh by fixing all warnings and most errors. There are now only a few remaining (low-priority, mostly "info") lints from ShellCheck.

QA/Testing:

make test

[hyperupcall]

@inkarkat Is there anything else that needa to be done? Or just waiting to be merged

[inkarkat]

@hyperupcall This PR is looking good, relax ;-)

I've suggested to @karbassi to first do a release (last one's been 4 years ago!), and then incorporate more outstanding PRs, followed by another release shortly thereafter. But I don't know whether he has the time, or first wants to add more maintainers and document the release process?!

[hyperupcall]

@inkarkat Ah, I understand, yeah it's probably best to do a release first. I suppose we'll see what happens