knot-resolver*_6: move extraFeatures to the -manager part
- reduces one layer of wrapper scripts
- reduces one layer of derivation in case of `extraFeatures = true`
- consequently overriding `extraFeatures = true` is simpler,
and the nix code itself got simpler as well
(cherry picked from commit dd21a60a31b681c692554dc11c403da6b1f1e13b)