This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Eliminate dependency on io_thread_get().

change the _delay API doc so that it matches its implementation behavior.

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

Replaces GLib's GError.

Remove GLib dependency from some headers.

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.

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

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

This method is used to reduce the delay of commands issued to the
shout plugin.

drain() is the opposite of cancel(): it waits until all data in the
buffer has finished playing.  Instead of implicitly draining in the
close() method like the ALSA plugin has been doing it forever, let the
output thread decide whether to drain or to cancel.

With these methods, an output plugin can allocate some global
resources only if it is actually enabled.  The method enable() is
called after daemonization, which allows for more sophisticated
resource allocation during that method.