Replaces decoder_data() and others.

Replaces decoder_initialized().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

Reduce header dependencies.

Merge duplicate data.

Clean up the process before exiting.

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.