nixos/sane: allow to disable enabled-by-default plugins

use case: disabling v4l plugin because I don't use my webcam as a
scanner.

nixos/modules/services/hardware/sane.nix

@@ -32,7 +32,7 @@ let
   };
 
   backends = [ pkg netConf ] ++ optional config.services.saned.enable sanedConf ++ config.hardware.sane.extraBackends;
-  saneConfig = pkgs.mkSaneConfig { paths = backends; };
+  saneConfig = pkgs.mkSaneConfig { paths = backends; inherit (config.hardware.sane) disabledDefaultBackends; };
 
   enabled = config.hardware.sane.enable || config.services.saned.enable;
 
@@ -75,6 +75,16 @@ in
       example = literalExample "[ pkgs.hplipWithPlugin ]";
     };
 
+    hardware.sane.disabledDefaultBackends = mkOption {
+      type = types.listOf types.str;
+      default = [];
+      example = [ "v4l" ];
+      description = ''
+        Names of backends which are enabled by default but should be disabled.
+        See <literal>$SANE_CONFIG_DIR/dll.conf</literal> for the list of possible names.
+      '';
+    };
+
     hardware.sane.configDir = mkOption {
       type = types.str;
       internal = true;