Use the C++ API.

Optimize the ProxyDatabase by invoking "list" on the peer, instead of
visiting all songs.

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

Let the database plugin do the match.

Merge locate_song_search() and locate_song_match().

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

This finally enables the new embedded CUE sheet code: when a song file
contains a playlist, it is printed in the "lsinfo" output, so clients
get to know about this.

The protocol mandates that playlist URIs are absolute (i.e. full URI
relative to the music directory, not relative to the parent
directory).  This adds the parameter "directory" to the "playlist"
visitor method.

Use it in handle_lsinfo(), and eliminate some duplicate code.

Add nonnull attributes and fix all callers.

Use this struct for db_walk().

Allow calling findAddIn() without a client object.

Allocate a player_control object where needed, and pass it around.
Each "client" object is associated with a "player_control" instance.

This prepares multi-player support.

In findAddInDirectory(), call playlist_append_song() directly, to have
some more type checking.

The only "return" statement always returns 0.

This fixes a regression in the patch "return multiple tag values per
song": even when the song has values for the specified tag type, the
empty string gets added to the set, because the "return" was removed.
This patch adds a flag which remembers whether at least one value was
found.

When collecting tag values for the result set, add all of a song's tag
values of the searched type.  This affects the "list" command.
Previously, "list" only considered the first tag value of a song.