trascode: reinit audio filters if audio format changes in midstream
faad: try to guess channel count if we can, and don't set it to 0 in Open

Ref #9200
avcodec: use context->channels in audio encoding

Input and output channel count should be anyway the same and it's
hopefully littlebit clearer than fmt_in and fmt_out mixups.
avcodec: use correct buffer size in fill_audio_frame
aout: avoid genmf
aout: remove Makefile recursion
sd: avoid genmf
sd: remove recursion
pulse: move helper library back to audio_outout/