When generating values for the services.nsd.zones attribute using values
from pkgs, we'll run into an infinite recursion because the nsd module
has a condition on the top-level definition of nixpkgs.config.

While it would work to push the definition a few levels down, it will
still only work if we don't use bind tools for generating zones.

As far as I could see, Python support for BIND seems to be only needed
for the dnssec-* tools, so instead of using nixpkgs.config, we now
directly override pkgs.bind instead of globally in nixpkgs.

To illustrate the problem with a small test case, instantiating the
following Nix expression from the nixpkgs source root will cause the
mentioned infinite recursion:

  (import ./nixos {
    configuration = { lib, pkgs, ... }: {
      services.nsd.enable = true;
      services.nsd.zones = import (pkgs.writeText "foo.nix" ''
        { "foo.".data = "xyz";
          "foo.".dnssec = true;
        }
      '');
    };
  }).vm

With this change, generating zones via import-from-derivation is now
possible again.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @pngwjpgh

While at it (see previous commit), using attrNames in combination with
length is a bit verbose for checking whether the filtered attribute set
is empty, so let's just compare it against an empty attribute set.

Signed-off-by: aszlig <aszlig@nix.build>