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.
If nothing has changed since the last save, don't save the state
file. Saving will spin up the hard drive, which is undesirable on
hosts where MPD is idling in background.
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.
