dnscrypt-proxy service: change default upstream resolver

Previously, the cisco resolver was used on the theory that it would
provide the best user experience regardless of location. The downsides
of cisco are 1) logging; 2) missing supoprt for DNS security extensions.

The new upstream resolver is located in Holland, supports DNS security,
and *claims* to not log activity. For users outside of Europe, this will
mean reduced performance, but I believe it's a worthy tradeoff.

nixos/modules/services/networking/dnscrypt-proxy.nix

···
         '';
       };
       resolverName = mkOption {
-        default = "cisco";
+        default = "dnscrypt.eu-nl";
         type = types.nullOr types.string;
         description = ''
           The name of the upstream DNSCrypt resolver to use. See
-          <filename>${resolverListFile}</filename> for alternative resolvers
-          (e.g., if you are concerned about logging and/or server
-          location).
+          <filename>${resolverListFile}</filename> for alternative resolvers.
+          The default resolver is located in Holland, supports DNS security
+          extensions, and claims to not keep logs.
         '';
       };
       customResolver = mkOption {