{v,a}f_showinfo: print frame side data
avconv: remove unused nb_frames_dup

Frame duplication now happens in vf_fps.
avconv: explicitly report when the muxing overhead is unknown
avconv: rewrite output data size tracking

Store a variable per OutputStream instead of globals for
audio/video/extradata. This makes the code simpler and cleaner and fixes
2pass with multiple output streams.
avconv: split printing the final statistics into a separate function
avconv: print verbose per-stream transcoding statistics
avconv: don't warn on multiple frames per packet for codecs that expect it