Remove dependency on ReplayGain global variables.

Eliminates Complete().

Previously, volume was applied twice: once by PcmVolume, and again by
the hardware mixer.

For easier state management inside filter plugins.

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

API simplification.  We can now avoid abusing a "size_t*" as
additional return value.

Prepare for adding state.

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

Replaces GLib's GError.