bug#16913: Please extend case-fold-search on-the-fly toggling

[Drew Adams]

> > Are you sure no one might want to use the global binding of M-c
> > to capitalize words in the minibuffer?  While redefining it
> > for read-regexp is fine, we need to allow disabling it,
> > so I suggest to add ‘(defvar-keymap read-regexp-map …’.
> 
> It seemed unlikely to me, and it makes the definition much simpler
> (because of closures), so I didn't make a map for this.
> 
> I.e., I don't think anybody is going to bother to change this binding.

Too bad.  Short-sighted.