This reverts commit a577944a.

Will be replaced by new options for the old commands "search" and
"find".

This command was removed by commit 206392ad (MPD 0.16), even though it
was been proven useful for some very simple clients.  On request, I
add it to the protocol again.

Replaces GLib's GError.

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.