Allow the plugin to allocate the GetSong() return value.

Duplicate the music_dir variable: one encoded in UTF-8, and another
one using the configured filesystem character set.  This fixes an
ancient MPD bug.

Implements error checking for the gmtime_r() call, which was missing
in two code locations.

Merge locate_song_search() and locate_song_match().

Fold the case during construction, without having to create another
copy.

Prepare to use the DatabasePlugin C++ API directly.

Prepare for splitting command.c.

Search and add search result to a stored playlist.

Requires UNIX domain socket connection.

To pass ack values around.

Make sure we get a UTF-8 encoded string.

Use ACK_ERROR_PERMISSION instead of ACK_ERROR_NO_EXIST.

An empty string is obviously not a valid integer.

Work around a confusing error message.