audiostep: Implemented filen() method to be used for asyncore or similar

Signed-off-by: David Jander <djander@gmail.com>
audiostep: Implemented minimal write function to be used with asycore

push_more_audio_data() will loop over main_iteration() until at least some
audio data has been written to the audio device.

Signed-off-by: David Jander <djander@gmail.com>