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

···
   };
 
   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;
 
···
         </para></note>
       '';
       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 {

pkgs/applications/graphics/sane/config.nix

···
 { lib, stdenv }:
 
-{ paths }:
+{ paths, disabledDefaultBackends ? [] }:
 
 with lib;
-let installSanePath = path: ''
+let
+installSanePath = path: ''
       if [ -e "${path}/lib/sane" ]; then
         find "${path}/lib/sane" -maxdepth 1 -not -type d | while read backend; do
           symlink "$backend" "$out/lib/sane/$(basename "$backend")"
···
         done
       fi
     '';
+    disableBackend = backend: ''
+      grep -q '${backend}' $out/etc/sane.d/dll.conf || { echo '${backend} is not a default plugin in $SANE_CONFIG_DIR/dll.conf'; exit 1; }
+      substituteInPlace $out/etc/sane.d/dll.conf --replace '${backend}' '# ${backend} disabled in nixos config'
+    '';
 in
 stdenv.mkDerivation {
   name = "sane-config";
···
     }
 
     mkdir -p $out/etc/sane.d $out/etc/sane.d/dll.d $out/lib/sane
-  '' + concatMapStrings installSanePath paths;
+  ''
+  + (concatMapStrings installSanePath paths)
+  + (concatMapStrings disableBackend disabledDefaultBackends);
 }