bug#8856: 24.0.50; regression: special-display-frame is no longer dedicated

[Drew Adams]

> What is happening now is that the first `C-g' does not remove 
> Completions (e.g. delete the frame) - a second `C-g' is needed.

And sometimes a second C-g does nothing either.
And sometimes C-g replaces buffer *Completions* in its frame with another buffer
(e.g. *scratch*), leaving the frame there.

I don't have a recipe to reproduce these latter symptoms.  But perhaps this is
all due to the window not being dedicated.