av_log: change how ffmpeg log messages are formatted

Don't print the ffmpeg context pointer as %p. This is usually useless
and confusing. Prefix all messages with "ffmpeg" to make clear that
ffmpeg is printing these messages, and not us.

If libavcodec is from Libav, use "libav" as prefix instead. (In theory,
FFmpeg/Libav libraries could be mixed, but I don't think that is
actually possible in practice.)