Declare two named elements.  An enum should not be used as an array
index, as this is error prone.

Eliminates Complete().

Move code from replay_gain_get_real_mode().

SimpleDatabase::Visit() always throws exception when visiting a song.
Adding a song to the queue always results in a "No such directory"
error response, although the song is successfully added.  This
behaviour was introduced by commit
3ff728ab.  This patch fixes this.

https://bugs.musicpd.org/view.php?id=4601

Speed up container_scan() again, by eliminating the need to call
scan_file() for each item.

Broken by commit 0abee77e

Got lost in 0abee77e

Catch ENOTDIR exceptions and in that case, try all matching decoder
plugins which implement the "container_scan" method.

 https://bugs.musicpd.org/view.php?id=4561

gcc 4.8 doesn't have full C++14 support, but it's enough for MPD.

Replaces decoder_data() and others.