This enforces the "not nullptr" rule.

See commit 71f0ed8b

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

An explicit destructor prevents usage of implicit move operators, even
if it's empty.  Therefore, declaring a defaulted destructor with GCC
attribute "noinline" does what we want without preventing those
implicit operators.

Eliminate one temporary allocation.

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

Reduce bloat.

The UPnP database plugin can now show relative song URIs for remote
songs.

Prepare for UPnP songs that retain there database identity.

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 regression from commit e96779de.

Finally restores an important feature that has been broken for several
months when the PlayerThread started working with Song copies instead
of pointers to the Queue's Song instances (commit e96779de).

Move code from class Instance.

Remove a GLib dependencies from class EventLoop and DatabaseLock.

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.

The signal_check library went out of order when we started using the
GLib main loop.  Convert the SIGHUP handler to use event_pipe instead.

Most of these functions were obsoleted when we switched to threaded
MPD.

"LOG_H" is a macro which is also used by ffmpeg/log.h.  This is
ffmpeg's fault, because short macros should be reserved for
applications, but since it's always a good idea to choose prefixed
macro names, even for applications, we are going to do that in MPD.

Try to only include headers which are really needed.  We should
particularly check all "headers including other headers".  The
long-term goal is to have a manageable, small API for plugins
(decoders, output) without so many mpd internals cluttering the
namespace.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7319 09075e82-0dd4-0310-85a5-a0d7c8717e4f