pkgs/nsd: Allow to easily override the package.

Allowing to use nixpkgs config to provide different defaults is not
going to help us here, so we would like to use nsd.override {} in order
to supply the correct options in the module.

Eventually removing the nixpkgs config option would make sense here as
well.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>