nixos/nsd: Allow to configure root zone

When trying to run NSD to serve the root zone, one gets the following
error message:

error: illegal name: '.'

This is because the name of the zone is used as the derivation name for
building the zone file. However, Nix doesn't allow derivation names
starting with a period.

So whenever the zone is "." now, the file name generated is "root"
instead of ".".

I also added an assertion that makes sure the user sets
services.nsd.rootServer, otherwise NSD will fail at runtime because it
prevents serving the root zone without an explicit compile-time option.

Tested this by adding a root zone to the "nsd" NixOS VM test.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @hrdinka, @qknight