x11 module: don't restart the display manager indefinitely

If the display managers crashes continuously in loops it prevents the
user from switching to the console and try to fix things. Especially
when using the "auto" display manager it can happen quite easily.

nixos/modules/services/x11/xserver.nix

@@ -529,6 +529,10 @@ in
           Restart = "always";
           RestartSec = "200ms";
           SyslogIdentifier = "display-manager";
+          # Stop restarting if the display manager stops (crashes) 3 times
+          # in 3 seconds.
+          StartLimitInterval = "3s";
+          StartLimitBurst = "3";
         };
       };