Preparing for FFmpeg 3.1 support.

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

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

Revert commit 70495aad by rewriting it.  Turns out, in old FFmpeg
versions, copying the AVPacket is necessary.

Don't scan tag items if the handler doesn't implement the tag()
method.

This is required if a stream ands without another chained FLAC file.

It's always 0.

If the duration is unknown, pass SignedSongTime::Negative(), as
documented for decoder_initialized().