Introduce a setting 'audio' in the config file to disable audio.

Currently this setting cannot yet be set in the userinterface config.
Having this option available, allows us to not call Mix_OpenAudio
if audio is completely disabled, so the audio subsystem is not initialized
at all.
Merge remote-tracking branch 'mainline/master'
Capsule the audio processing better into their own functions.
Shrink scope of variables.
remove unneeded assignments.
PowerManager: complete init list.
Add Gallaecio to mailmap file.
Merge pull request #313 from stefanbeller/master

Introduce a setting 'audio' in the config file to disable audio.