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.

Worst-case allocation using MAX_CHANNELS.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Return the pointer given by PcmBuffer::Get() instead of reaching into
the PcmBuffer object.

Fixes a clang warning.