Openldap needs a directory in /var/run to start, and it seems it's clean at every boot.

svn path=/nixos/trunk/; revision=26840

modules/services/databases/openldap.nix

@@ -50,6 +50,10 @@ in
         startOn = "filesystem";
         daemonType = "fork";
 
+        preStart =
+          ''
+            mkdir -p /var/run/slapd
+          '';
         exec = "${openldap}/libexec/slapd -f ${configFile}";
       };