[PiSink] Fix some issues with buffering and multichannel
AE: add settings to enable ac3 transcode and upmix via ac3 transcoding
ActiveAE: implement settings for ac3 transcode and upmix
[rbp] Disable ac3transcode by default
AE: kill macros in sink factory, remove null sink logic
ActiveAE: try other sinks in case of create fails
[rbp] Allow ALSA to be chosen in addition to Pi sink

Needs --enable-alsa in ./configure step and alsa support on platform