* lisp/isearch.el: No visual feedback for default search mode
During an isearch where character-folding is the default, we don't
want to take up minibuffer space just to tell the user that
"Char-fold " is on. The same goes for other modes, if the user
changes the default. In contrast, if the user toggles OFF the
default mode, they should see "Literal", to distinguish it from
the default mode.
(isearch--describe-regexp-mode): Return "" if describing the
default mode, and return "literal " if describing a plain search
and it is not default.