slim: Only enable if the X server is enabled

http://hydra.nixos.org/build/6494972

Disabling slim revealed that services.xserver.displayManager.job had
an incorrect type, so fixed that as well.

nixos/modules/services/x11/display-managers/default.nix

···
         };
       };
 
-      job = mkOption {
-        default = {};
-        type = types.uniq types.optionSet;
-        description = "This option defines how to start the display manager.";
+      job = {
 
-        options = {
+        preStart = mkOption {
+          type = types.lines;
+          default = "";
+          example = "rm -f /var/log/my-display-manager.log";
+          description = "Script executed before the display manager is started.";
+        };
 
-          preStart = mkOption {
-            default = "";
-            example = "rm -f /var/log/my-display-manager.log";
-            description = "Script executed before the display manager is started.";
-          };
-
-          execCmd = mkOption {
-            example = "${pkgs.slim}/bin/slim";
-            description = "Command to start the display manager.";
-          };
-
-          environment = mkOption {
-            default = {};
-            example = { SLIM_CFGFILE = /etc/slim.conf; };
-            description = "Additional environment variables needed by the display manager.";
-          };
+        execCmd = mkOption {
+          type = types.uniq types.string;
+          example = "${pkgs.slim}/bin/slim";
+          description = "Command to start the display manager.";
+        };
 
-          logsXsession = mkOption {
-            default = false;
-            description = ''
-              Whether the display manager redirects the
-              output of the session script to
-              <filename>~/.xsession-errors</filename>.
-            '';
-          };
+        environment = mkOption {
+          default = {};
+          example = { SLIM_CFGFILE = /etc/slim.conf; };
+          description = "Additional environment variables needed by the display manager.";
+        };
 
+        logsXsession = mkOption {
+          type = types.bool;
+          default = false;
+          description = ''
+            Whether the display manager redirects the
+            output of the session script to
+            <filename>~/.xsession-errors</filename>.
+          '';
         };
 
       };

nixos/modules/services/x11/display-managers/kdm.nix

···
     services.xserver.displayManager.slim.enable = false;
 
     services.xserver.displayManager.job =
-      { execCmd =
+      { execCmd = mkFixStrictness
           ''
             mkdir -m 0755 -p /var/lib/kdm
             chown kdm /var/lib/kdm

nixos/modules/services/x11/display-managers/lightdm.nix

···
   };
 
   config = mkIf cfg.enable {
+
+    services.xserver.displayManager.slim.enable = false;
+
     services.xserver.displayManager.job = {
       logsXsession = true;
 
       # lightdm relaunches itself via just `lightdm`, so needs to be on the PATH
-      execCmd = ''
+      execCmd = mkFixStrictness ''
         export PATH=${lightdm}/sbin:$PATH
         ${lightdm}/sbin/lightdm --log-dir=/var/log --run-dir=/run --config=${lightdmConf}
       '';

nixos/modules/services/x11/display-managers/slim.nix

···
     services.xserver.displayManager.slim = {
 
       enable = mkOption {
-        default = true;
+        default = config.services.xserver.enable;
         description = ''
           Whether to enable SLiM as the display manager.
         '';