davfs2: create user/group davfs2 if not specified in the configuration

* Add options:
  - enable
  - davUser (default: "davfs2")
  - davGroup (default: "davfs2)
* Add davfs2 user or group if they are not specified in the
configuration

nixos/modules/module-list.nix

@@ -401,6 +401,7 @@
   ./services/monitoring/zabbix-agent.nix
   ./services/monitoring/zabbix-server.nix
   ./services/network-filesystems/cachefilesd.nix
+  ./services/network-filesystems/davfs2.nix
   ./services/network-filesystems/drbd.nix
   ./services/network-filesystems/glusterfs.nix
   ./services/network-filesystems/kbfs.nix

nixos/modules/services/network-filesystems/davfs2.nix

@@ -0,0 +1,74 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.services.davfs2;
+  cfgFile = pkgs.writeText "davfs2.conf" ''
+    dav_user ${cfg.davUser}
+    dav_group ${cfg.davGroup}
+    ${cfg.extraConfig}
+  '';
+in
+{
+  options.services.davfs2 = {
+    enable = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to enable davfs2.
+      '';
+    };
+
+    davUser = mkOption {
+      type = types.string;
+      default = "davfs2";
+      description = ''
+        When invoked by root the mount.davfs daemon will run as this user.
+        Value must be given as name, not as numerical id.
+      '';
+    };
+
+    davGroup = mkOption {
+      type = types.string;
+      default = "davfs2";
+      description = ''
+        The group of the running mount.davfs daemon. Ordinary users must be
+        member of this group in order to mount a davfs2 file system. Value must
+        be given as name, not as numerical id.
+      '';
+    };
+
+    extraConfig = mkOption {
+      type = types.lines;
+      default = "";
+      example = ''
+        kernel_fs coda
+        proxy foo.bar:8080
+        use_locks 0
+      '';
+      description = ''
+        Extra lines appended to the configuration of davfs2.
+      ''  ;
+    };
+  };
+
+  config = mkIf cfg.enable {
+    environment.systemPackages = [ pkgs.davfs2 ];
+    environment.etc."davfs2/davfs2.conf".source = cfgFile;
+
+    users.extraGroups = optionalAttrs (cfg.davGroup == "davfs2") (singleton {
+      name = "davfs2";
+      gid = config.ids.gids.davfs2;
+    });
+
+    users.extraUsers = optionalAttrs (cfg.davUser == "davfs2") (singleton {
+      name = "davfs2";
+      createHome = false;
+      group = cfg.davGroup;
+      uid = config.ids.uids.davfs2;
+      description = "davfs2 user";
+    });
+  };
+
+}