https://bugs.musicpd.org/view.php?id=4577

An empty packet would be a command for avcodec_send_packet() to
finalize the codec.

Fixes https://bugs.musicpd.org/view.php?id=4588

Let std::unique_ptr manage both the InputStream and the WavpackInput.

This function exists since FFmpeg 3.1.  Fix a build failure with
FFmpeg 3.0.

These bug reports describe problems with some FFmpeg codecs:

 https://bugs.musicpd.org/view.php?id=4564
 https://bugs.musicpd.org/view.php?id=4568
 https://bugs.musicpd.org/view.php?id=4572

According to the FFmpeg bug tracker, a call to
avcodec_parameters_to_context() is required after
avcodec_alloc_context3():

 https://trac.ffmpeg.org/ticket/5781

This requirement was previously undocumented.

Turns out the libavcodec version numbers are not linear;  the feature
was added in FFmpeg 3.1 commit 998e1b8, libavcodec 57.14.0; but FFmpeg
3.0 has version 57.48.101.  Ouch!

https://bugs.musicpd.org/view.php?id=4558

Was accidently added by commit cafc266e

The ReSIDBuilder constructor cannot fail.

Eliminate duplicate SidTune construction.

Remove our own songlength database parser.

Eliminates duplicate ParseContainerPath() call.

This commit suppresses the remaining deprecation warnings with FFmpeg 3.1.

The AVCodecContext attribute is deprecated.

Preparing for FFmpeg 3.1 support.