nixos/tor: don't do privoxy stuff by default

It's very surprising that services.tor.client.enable would set
services.privoxy.enable.  This violates the principle of least
astonishment, because it's Privoxy that can integrate with Tor, rather
than the other way around.

So this patch moves the Privoxy Tor integration to the Privoxy module,
and it also disables it by default.  This change is documented in the
release notes.

Reported-by: V <v@anomalous.eu>

nixos/modules/services/networking/privoxy.nix

@@ -8,15 +8,22 @@ let
 
   cfg = config.services.privoxy;
 
-  confFile = pkgs.writeText "privoxy.conf" ''
+  confFile = pkgs.writeText "privoxy.conf" (''
     user-manual ${privoxy}/share/doc/privoxy/user-manual
     confdir ${privoxy}/etc/
     listen-address  ${cfg.listenAddress}
     enable-edit-actions ${if (cfg.enableEditActions == true) then "1" else "0"}
     ${concatMapStrings (f: "actionsfile ${f}\n") cfg.actionsFiles}
     ${concatMapStrings (f: "filterfile ${f}\n") cfg.filterFiles}
+  '' + optionalString cfg.enableTor ''
+    forward-socks4a / ${config.services.tor.client.socksListenAddressFaster} .
+    toggle 1
+    enable-remote-toggle 0
+    enable-edit-actions 0
+    enable-remote-http-toggle 0
+  '' + ''
     ${cfg.extraConfig}
-  '';
+  '');
 
 in
 
@@ -72,6 +79,15 @@ in
         '';
       };
 
+      enableTor = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether to configure Privoxy to use Tor's faster SOCKS port,
+          suitable for HTTP.
+        '';
+      };
+
       extraConfig = mkOption {
         type = types.lines;
         default = "" ;