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

@@ -239,20 +239,17 @@ in
         };
       };
 
-      job = mkOption {
-        default = {};
-        type = types.uniq types.optionSet;
-        description = "This option defines how to start the display manager.";
-
-        options = {
+      job = {
 
         preStart = mkOption {
+          type = types.lines;
           default = "";
           example = "rm -f /var/log/my-display-manager.log";
           description = "Script executed before the display manager is started.";
         };
 
         execCmd = mkOption {
+          type = types.uniq types.string;
           example = "${pkgs.slim}/bin/slim";
           description = "Command to start the display manager.";
         };
@@ -264,6 +261,7 @@ in
         };
 
         logsXsession = mkOption {
+          type = types.bool;
           default = false;
           description = ''
             Whether the display manager redirects the
@@ -278,6 +276,4 @@ in
 
   };
 
-  };
-
 }

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

@@ -128,7 +128,7 @@ in
     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

@@ -89,11 +89,14 @@ in
   };
 
   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

@@ -45,7 +45,7 @@ in
     services.xserver.displayManager.slim = {
 
       enable = mkOption {
-        default = true;
+        default = config.services.xserver.enable;
         description = ''
           Whether to enable SLiM as the display manager.
         '';