Merge: gamnit audio: control sound channels and don't crash if there's no sound card

Desktop clients can now control playing sounds with the new services from SDL2 mixer. The equivalent was already implemented for Android but not GNU/Linux. The new services still need to be fully exposed in the `app::audio` API.

This PR also removes the requirement to have a working sound card to launch a gamnit game.

Pull-Request: #2537
Reviewed-by: Jean Privat <jean@pryen.org>