bug#42829: save-some-buffers should learn to "just let go"

[Juri Linkov]

> C-x s says
>
>   Save file ...documentation.txt? (y, n, !, ., q, C-r, d or C-h)
>
> C-h then says
>
>   Type SPC or ‘y’ to save the current buffer;
>   DEL or ‘n’ to skip the current buffer;
>   RET or ‘q’ to give up on the save (skip all remaining buffers);
>   C-g to quit (cancel the whole command);
>   ! to save all remaining buffers;
>   C-r to view this buffer;
>   d to view changes in this buffer;
>   or . (period) to save the current buffer and exit.
>
> Problem is: C-r won't "let go of the user".
>
> He can't just visit the buffer (to say undo his changes there.)
> He is still trapped at the prompt. No C-x o won't free him temporarily.

This means that map-y-or-n-p (used by save-some-buffers) should be rewritten
to rely on read-char-from-minibuffer.