ffplay: restructure audio stream opening code

Preparation for -af support.

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: use frame->pts if available for setting the audio clock

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: use refcounted frames for audio

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: add -af option

Based on a patch by Stefano Sabatini <stefasab@gmail.com>:
http://ffmpeg.org/pipermail/ffmpeg-devel/2013-February/138452.html

Signed-off-by: Marton Balint <cus@passwd.hu>
Merge remote-tracking branch 'cus/stable'

* cus/stable:
  ffplay: add -af option
  ffplay: use refcounted frames for audio
  ffplay: use frame->pts if available for setting the audio clock
  ffplay: restructure audio stream opening code

Merged-by: Michael Niedermayer <michaelni@gmx.at>