Fixed memory leak in error handler.

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.

Allow input plugins to configure with an "input" block in mpd.conf.
Also allow the user to disable a plugin completely.

Create a sub directory for input plugins.

Start to separate private from public input_stream API.

The open_stream() method opens the input_stream.  This allows the
archive plugin to do its own initialization, and it also allows it to
use input_stream.data.  We can remove input_stream.archive now, which
was unnatural to have in the first place.

This way, plugins can manipulate the plugin pointer during open().