Fixes the bogus duration of the last track in a CUE sheet.

Lists files and directories.  Supports storage plugins.

Replaces map_to_relative_path() from Mapper.cxx.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Reduces overhead because we need to manage only one memory allocation.
According to valgrind/massif, we save 7%.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

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

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

If a song with an absolute path points inside the music directory,
print only the relative part.  This happens when partial songs from a
playlist file were loaded.

On WIN32, "%F" and "%T" are not implemented.

The only "return" statement always returns 0.