Improve initial seek by not reading/decoding the first frame before
checking for the seek command.

When seeking to the beginning of a packet, skip the samples that come
before the desired time stamp.

Ask FFmpeg to seek to the next packet boundary *before* the seek
position, so we don't miss audio data.  Now we get too much, but we'll
solve that in the next commit.

The old struct config_param remains only for top-level string options.

Use the duration of the stream we're actually decoding - not the
"global" attribute AVFormatContext::duration which may differ.

Fixes regression by commit 7c8038e2