Reduce overhead.  LightSong can always point to the "real" allocated
string.

The Song class is only used for database songs now.  A Song without a
Directory is not possible anymore.

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.

Fixes wrong object URIs with duplicate base names.

Initial support for "detached" songs that come from the database, but
are private copies.

decoder_is_current_song() now recognizes different instances of the
same physical song.

Added attributes start_ms, end_ms.  This allows us to address a
portion of a song file (important for CUE support).  There is no
support yet for storing these attributes in the state file.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

Moved all the code which depends on the decoder plugins to a separate
source.  That allows leaner test programs.

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

"ls" is a bad name for a library which parses URIs.  We'll move the
rest of the "ls" library later.

The decoder_plugin struct is used by both the MPD core and the decoder
plugin implementations.  Move it to a shared header file, to minimize
header dependencies.

Removed unused includes.