User acknowledgement not required for critical-urgency notifications

[intuited]

According to the Urgency Levels section of [the spec](http://www.galago-project.org/specs/notification/0.9/x320.html), notifications sent with critical urgency should not expire:

> Critical notifications should not automatically expire, as they are things that the user will most likely want to know about.
> They should only be closed when the user dismisses them, for example, by clicking on the notification.

I'm not seeing any difference with urgent messages created with `notify-send`: they are, by default, displayed for 10 seconds just like normal ones. For example, after issuing the command

    notify-send -u critical Testing "this is a test of the emergency broadcast system."

, I will see the overlay popup displaying this message for 10 seconds.

Running ubuntu 10.10; notify-osd v0.9.29-0ubuntu3