lint: Fix all ShellCheck errors and most warnings

[hyperupcall]

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

• [x] Fork the repository and create your branch from master.
• [x] 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.
• [x] 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