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

[Juri Linkov]

> So I've now added an `M-c' command to `read-regexp' and made `rgrep'
> heed that in Emacs 29.

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 …’.