modules/config/networking.nix: recognize whether a local DNS resolver is available

resolvconf prefers a locally running BIND resolver over the forwarders; we just
have to tell it whether we have one or not. We use 'config.services.bind.enable'
to make that decision, assuming that people are not going to configure a local
BIND that won't respond to queries on 127.0.0.1. If we run into such a (weird)
case, then we'll need to introduce another variable for that purpose which can
be set independently from 'config.services.bind.enable'.

modules/config/networking.nix

@@ -68,6 +68,9 @@ in
             # Invalidate the nscd cache whenever resolv.conf is
             # regenerated.
             libc_restart='${pkgs.upstart}/sbin/start invalidate-nscd'
+          '' + optionalString config.services.bind.enable ''
+            # This hosts runs a full-blown DNS resolver.
+            name_servers='127.0.0.1'
           '' );
         target = "resolvconf.conf";
       }