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

nixos/tests/nsd.nix

@@ -41,6 +41,7 @@ in import ./make-test.nix ({ pkgs, ...} : {
         { address = "dead:beef::1"; prefixLength = 64; }
       ];
       services.nsd.enable = true;
+      services.nsd.rootServer = true;
       services.nsd.interfaces = lib.mkForce [];
       services.nsd.zones."example.com.".data = ''
         @ SOA ns.example.com noc.example.com 666 7200 3600 1209600 3600
@@ -55,6 +56,11 @@ in import ./make-test.nix ({ pkgs, ...} : {
         @ A 9.8.7.6
         @ AAAA fedc::bbaa
       '';
+      services.nsd.zones.".".data = ''
+        @ SOA ns.example.com noc.example.com 666 7200 3600 1209600 3600
+        root A 1.8.7.4
+        root AAAA acbd::4
+      '';
     };
   };
 
@@ -86,6 +92,9 @@ in import ./make-test.nix ({ pkgs, ...} : {
 
         assertHost($_, "a", "deleg.example.com", qr/address 9.8.7.6$/);
         assertHost($_, "aaaa", "deleg.example.com", qr/address fedc::bbaa$/);
+
+        assertHost($_, "a", "root", qr/address 1.8.7.4$/);
+        assertHost($_, "aaaa", "root", qr/address acbd::4$/);
       };
     }
   '';