nixos/resolvconf: allow disabling

For systems without internet connections, it doesn't make sense to
require the existence of an /etc/resolv.conf file to disable
resolvconf, so let's expose networking.resolveconf.enable as a public
option that can be set to false.

nixos/modules/config/resolvconf.nix

@@ -47,8 +47,8 @@ in
 
       enable = mkOption {
         type = types.bool;
-        default = false;
-        internal = true;
+        default = !(config.environment.etc ? "resolv.conf");
+        defaultText = literalExpression ''!(config.environment.etc ? "resolv.conf")'';
         description = ''
           DNS configuration is managed by resolvconf.
         '';
@@ -110,8 +110,6 @@ in
 
   config = mkMerge [
     {
-      networking.resolvconf.enable = !(config.environment.etc ? "resolv.conf");
-
       environment.etc."resolvconf.conf".text =
         if !cfg.enable then
           # Force-stop any attempts to use resolvconf