Rename our local copy of av_read_frame_flush to xbmc_read_frame_flush as suggested by elupus on PR2633.

That way it makes it clear the function is from xmbc and not ffmpeg.
[darwin] - revert the revert the revert - well fix the build for darwin broken in a7703fb74603192f087db90c60877eeed1097f66
DllAvCodec: map avcodec_encode_audio2
DllAvCodec: map avcodec_fill_audio_frame
DllAvCodec: map avcodec_free_frame.

This makes xbmc require FFmpeg 1.0