Apply mornfall's SANE patches

Fix sane-backends to generate udev rules, add a snapshot of sane-backends's unstable repo, and add a SANE nixos module

svn path=/nixos/trunk/; revision=30764

modules/module-list.nix

@@ -68,6 +68,7 @@
   ./services/hardware/bluetooth.nix
   ./services/hardware/hal.nix
   ./services/hardware/pcscd.nix
+  ./services/hardware/sane.nix
   ./services/hardware/udev.nix
   ./services/hardware/udisks.nix
   ./services/hardware/upower.nix

modules/services/hardware/sane.nix

@@ -0,0 +1,34 @@
+{ config, pkgs, ... }:
+
+with pkgs.lib;
+
+{
+
+  ###### interface
+
+  options = {
+
+    hardware.sane.enable = mkOption {
+      default = false;
+      description = "Enable support for SANE scanners.";
+    };
+
+    hardware.sane.snapshot = mkOption {
+      default = false;
+      description = "Use a development snapshot of SANE scanner drivers.";
+    };
+
+  };
+
+
+  ###### implementation
+
+    config = let pkg = if config.hardware.sane.snapshot
+                          then pkgs.saneBackendsSnapshot
+                          else pkgs.saneBackends;
+      in mkIf config.hardware.sane.enable {
+           environment.systemPackages = [ pkg ];
+           services.udev.packages = [ pkg ];
+      };
+
+}