slim: Work around broken PAM session handling

Previously logging in via SLiM more than once didn't work because SLiM
doesn't clean up its PAM session properly (that is, in a child rather
than in the parent). Thus the slim process becomes part of the user
session's cgroup, among other things. This patch causes SLiM to exit
after the session has finished, after which systemd will restart
display-manager.service.

Fixes NixOS/nixops#137.

nixos/modules/services/x11/xserver.nix

···
           '';
 
         script = "${cfg.displayManager.job.execCmd}";
+
+        serviceConfig = {
+          Restart = "always";
+          RestartSec = "200ms";
+        };
       };
 
     services.xserver.displayManager.xserverArgs =

pkgs/applications/display-managers/slim/default.nix

···
     [ # Allow the paths of the configuration file and theme directory to
       # be set at runtime.
       ./runtime-paths.patch
+
+      # Exit after the user's session has finished.  This works around
+      # slim's broken PAM session handling (see
+      # http://developer.berlios.de/bugs/?func=detailbug&bug_id=19102&group_id=2663).
+      ./run-once.patch
     ];
 
   preConfigure = "substituteInPlace CMakeLists.txt --replace /etc $out/etc --replace /lib $out/lib";

pkgs/applications/display-managers/slim/run-once.patch

···
+diff -ru -x '*~' slim-1.3.6-orig/app.cpp slim-1.3.6/app.cpp
+--- slim-1.3.6-orig/app.cpp	2013-10-15 11:02:55.629263422 +0200
++++ slim-1.3.6/app.cpp	2013-10-15 13:00:10.141210784 +0200
+@@ -816,7 +822,7 @@
+ 	StopServer();
+ 	RemoveLock();
+ 	while (waitpid(-1, NULL, WNOHANG) > 0); /* Collects all dead childrens */
+-	Run();
++	exit(OK_EXIT);
+ }
+ 
+ void App::KillAllClients(Bool top) {