[linux] add config file for env vars and suspend pulse if AE_SINK=alsa

[wsnipex]

This allows to put our supported environment variables in a config file userdata/envvars.conf.
Main use case is for using Alsa even when pulse is installed. If AE_SINK=alsa is specified, pulse will be suspended via pavucontrol when starting xbmc. After exit, pulse will be resumed.

If acceptable, I'd like to get this in for gotham, as it could fix a problem for xbmcbuntu.
We could install pulse by default for the desktop session without having to deal with its limitations(no HD audio).

@fritsch ping

[fritsch]

I see the usecase for xbmcbuntu and also for appliances like OpenELEC, that want to integrate "more than xbmc" in the future. It's okay from my side. For endusers it is a bit hard to need to edit yet another config file, though.

[t-nelson]

I'm really not interested in adding yet another place to configure xbmc.
Especially a platform specific one like this.

[wsnipex]

well, there is no other place for this atm. This just adds a unified way to pass our env vars, which can already be passed to xbmc.
Adding it to normal settings would require significant changes, if at all possible. I'm open for suggestions.

[topfs2]

Personally I think this is not something we should manage in xbmc. Its up to xbmcubuntu and openelec and others to have their own way to manage the env vars needed. I personally think we should just behave sanely as an application here, either we can connect to pulse or not.

I do understand the need for it but feels misplaced to have it in xbmc source IMO.