bug#21140: 25.0.50; Isearch "char-fold" by default

[Juri Linkov]

> There's another thread here about hiding that string from the prompt. I
> agree it's a little confusing, and char folding can be used without
> flashing it on the user. Somebody offered to do something about it, though
> I forget who.

The idea was to not display the search parameters that have default values,
i.e. if char folding is enabled by default then to not display “Char-fold”.
But the problem is to find a name complementary to char-folding
to display when the default char-folding is disabled by ‘M-s '’.

In case of case-folding the parameter names used in isearch messages are:
“case-sensitive” and “case-insensitive”.

By analogy, can we use: “char-sensitive” and “char-insensitive”?