Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.

This is like tag_dup(), but works with an input_stream object instead
of a file path.

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.

[mk: fixed whitespace errors; use delete_song() instead of
songvec_delete()]

Preparing for per-plugin configuration sections in mpd.conf.

Increase code readability, always use the wrapper functions instead of
calling the plugin method pointers directly.

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.