[Libosinfo] Debian & Ubuntu install scripts

[Zeeshan Ali (Khattak)]

While I was about to push these patches on behalf of Guiseppe, I realized some things that could be improved in the api:

* 'The 'supported/SUPPORTED' part of these API seems very redundant and makes identifiers way too long.
* get_injection_method should be plural form: get_injection_methods.

These patches fixes/improves these issues.