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

[Juri Linkov]

>> The idea was to add the capability to do case-insensitive matching to
>> all commands that read regexs. The list of simple commands one would
>> have to write could be quite long!
>
> Yup.  I don't know what the best way for `read-regexp' to communicate
> this back up to the caller, though.  Perhaps the easiest would be to
> just add a new optional parameter to `read-regexp' that would make it
> activate the `M-c' key binding, and then return a list that has both the
> regexp we read and whether the user used a case twiddling command.

Maybe `read-regexp' could modify some global variable?