Improve getopts and stderr message consistency in main

[roderickvd]

There is no definitive standard on how to format getopts strings.
Usage of curly brackets seems best practice by these standards:

• http://www.tfug.org/helpdesk/general/man.html
• https://developers.google.com/style/code-syntax

This is cosmetic only and was mostly extracted from #685.

[JasonLG1979]

Not really an "options" issue but more of an unexpected missing arg. There is no --help to list all possible args. Granted if you try librespot --help it will complain with error: Unrecognized option: 'help' and then proceed to list all possible args. It would be nice if librespot had a proper help (-h, --help) It could be a separate PR but it seems like an oversight to not have a help.

[roderickvd]

Here you go.