[dev] ingo vacation api

[Chuck Hagenbuch]

Quoting Duck <duck at obala.net>:

> I am writing an user profile application. Users will be able to set their
> vacation message to be show in their public profile page. I will like that
> the vacation is set event in their mail boxes if the user has our mail
> account. So I patch ingo api to allow set/disable the vacation filter. I
> don't now ingo so much, but looking at the code it should be done right.

Looks okay to me in general. I've committed it; the only changes I  
made were to split it into two methods (one for setting, one for  
disabling), and to remove the use of the Notification API and instead  
return errors directly, as an API should.

Thanks!

-chuck