nixos/bind: configurable "forward" setting

Sometimes it is preferable to configure forwarding only for bind
instead of relying on direct lookups.

This patch makes it possible to configure the forward setting to
either "first" (the default) or "only".

nixos/modules/services/networking/bind.nix

@@ -59,7 +59,7 @@ let
         listen-on-v6 { ${concatMapStrings (entry: " ${entry}; ") cfg.listenOnIpv6} };
         allow-query { cachenetworks; };
         blackhole { badnetworks; };
-        forward first;
+        forward ${cfg.forward};
         forwarders { ${concatMapStrings (entry: " ${entry}; ") cfg.forwarders} };
         directory "${cfg.directory}";
         pid-file "/run/named/named.pid";
@@ -151,6 +151,14 @@ in
         ";
       };
 
+      forward = mkOption {
+        default = "first";
+        type = types.enum ["first" "only"];
+        description = "
+          Whether to forward 'first' (try forwarding but lookup directly if forwarding fails) or 'only'.
+        ";
+      };
+
       listenOn = mkOption {
         default = [ "any" ];
         type = types.listOf types.str;