Reduce some overhead.  It is not necessary to copy the object.

av_free_packet() was deprecated in FFmpeg 3.0.

Fixes aborted "load" commands due to SongLoader failure.

Adds support for stream codecs which havn't been explicitly listed in
ffmpeg_mime_types.

For std::runtime_error.  See http://bugs.musicpd.org/view.php?id=4509

The Mutex and Cond constructors are only "constexpr" with glibc, and
this is what this #ifdef is about.

Backport of commit 459a812a

See http://bugs.musicpd.org/view.php?id=4511

Use this function instead of CheckError() when we already know an
error has occurred.