Fixes assertion failure in MultipleOutputs::Open() after seeking to a
failing song during playback.

Fixes an assertion failure in tag_ape_scan() which is because
decoder_input_stream_open() returns an "unready" InputStream.

playlist_file_print() already handles stored playlists.  spl_print()
is never called with a successful result.

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