nixos/nsd: Fix indentation/coding style.

For Nix, we indent using two spaces, but in this module somehow 4 spaces
were snuck in. Other than that, remoteControl and ratelimit are just
nested attribute sets, so we don't need to make another submodule type
for no particular reason.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
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>
nixos/networking: Fix typo in ipv6prefixLength.

Within the module it's referenced with an uppercase "P" and ipv6Address
also begins with an uppercase "A" after the "6", so let's make it
consistent.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
nixos/nsd: Improve support for journald/systemd.

Don't fork into the background and just log to stderr.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
nixos/tests: Add basic test for nsd module.

Currently only tests basic resource record lookup against IPv4 and IPv6.
Nothing special yet, but probably enough for most setups.

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