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

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