Add a watchdog log entry when a user turns the site into maintenance mode (aka. Site offline)

I have a site that I administer. I visted it today and noticed it was set to "Offline Mode". I'm unsure why or who did this.

I went in the watchdog, to see if I could see which user put it offline and more importantly when. To my surprise this important event does not exist in the log.

I have a feature request to add information relating the turning off of a site into the watchdog. Currently, we'd be able to log which user turns off the site and at what time with a single line in the code. I think this is an important event and which should get logged. Logging when it gets turned online is also the important flip side to this request.

Ideally I'd love to see the ability for the user who turns off the site, to include a message as to why they are shutting it off. This is not important though, as I realize asking for a feature like this, could delay the implementation in the wonderful world of core modifications.

With that said, I'm sure we can all agree that at a very minimum we should log who did this and more importantly when.