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

[Juri Linkov]

>> 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.

It's easy to toggle a (mini)buffer-local variable like toggling in isearch does.