For easier state management inside filter plugins.

The old struct config_param remains only for top-level string options.

Replaces GLib's GError.

These fixes were mostly generated with `codespell' [0] and manually
reviewed.

[0] http://git.profusion.mobi/cgit.cgi/lucas/codespell/

Make the audio_format argument non-const.  Allow the open() method to
modify it, to indicate that it wants a different input audio format
than the one specified.  Check that condition in chain_filter_open(),
and fail.

The filter API allows us to implement software volume as a pluggable
filter, and we will be able to integrate libraries like SoX.