Improved usage messages for actions

[chrysle]

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.

I have substituted all occurrences of ITEM# with NR and converted the usage messages to the suggested syntax. Remind me if I forgot something. I have also added another $errmsg variable for the addto action to avoid duplicating the usage message.

Fixes #401

[chrysle]

By the way, I wonder if support for 18.04 could be dropped by now, since it's EOL in April?

[inkarkat]

Yes, we should update to more recent Ubuntu versions, and the MacOS platforms have been completely broken for way too long. @karbassi will hopefully soon dedicate some time for that.

[chrysle]

Sorry. Completely forgot the usage in USAGE.md!