Enables using range-based "for".

Make it "const".

.. and move the plugins to src/encoder/plugins/.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

The MPD core will add this as a prefeix.

Migrate the remaining callers to FatalError().

Replaces GLib's GError.

Don't use macro magic, don't dereference the block_param.

Eliminate the evil goto.

This is necessary for Ogg packets that span more than one page.

Move the raw buffer to struct shout_data.

Make it a local variable instead.

Pass audio_output objects around instead of void pointers.  This will
give some more control to the plugin, and prepares for non-blocking
audio outputs.

Consistent naming.

Move the "extern" declarations from output_list.c, for more type
safety.

No caller needs to write.

In the "vorbis" plugin, this is a copy of the old flush() method,
while flush() gets a lot of code remove, it just sets the "flush" flag
and nothing else.  It doesn't start a new stream now, which should fix
a few problems in some players.

Added a new optional parameter for the shout plugin called "url".