programs.criu: add nixos option for installing criu + kernel flags

nixos/modules/module-list.nix

···
   ./programs/cdemu.nix
   ./programs/chromium.nix
   ./programs/command-not-found/command-not-found.nix
+  ./programs/criu.nix
   ./programs/dconf.nix
   ./programs/environment.nix
   ./programs/fish.nix

nixos/modules/programs/criu.nix

···
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let cfg = config.programs.criu;
+in {
+
+  options = {
+    programs.criu = {
+      enable = mkOption {
+        default = false;
+        description = ''
+          Install <command>criu</command> along with necessary kernel options.
+        '';
+      };
+    };
+  };
+  config = mkIf cfg.enable {
+    system.requiredKernelConfig = with config.lib.kernelConfig; [
+      (isYes "CHECKPOINT_RESTORE")
+    ];
+    boot.kernel.features.criu = true;
+    environment.systemPackages = [ pkgs.criu ];
+  };
+
+}

pkgs/os-specific/linux/kernel/common-config.nix

···
     WW_MUTEX_SELFTEST? n
   ''}
 
+  ${optionalString (features.criu or false)  ''
+    EXPERT y
+    CHECKPOINT_RESTORE y
+  ''}
+
+  ${optionalString ((features.criu or false) && (features.criu_revert_expert or true))
+    # Revert some changes, introduced by EXPERT, when necessary for criu
+  ''
+    RFKILL_INPUT? y
+    HID_PICOLCD_FB? y
+    HID_PICOLCD_BACKLIGHT? y
+    HID_PICOLCD_LCD? y
+    HID_PICOLCD_LEDS? y
+    HID_PICOLCD_CIR? y
+    DEBUG_MEMORY_INIT? y
+  ''}
+
   ${extraConfig}
 ''