make audio switchable on/off at start/build-time

[ghost]

Since 0.53, it's possible to launch engine even if there is a problem with the audio subsystem. However, I have not seen any way to launch daemon by disabling audio, or to compile audio support out.

I noticed this missing feature while trying to build&run older version, where daemon dies with the message: Warn: Couldn't load the default resource for N5Audio6SampleE.

[slipher]

Setting ALSOFT_DRIVERS=null in the environment seems to work. This selects the "null" backend.