The previous patch fixes the problem with y-or-n-p reported in bug#17272.

But to fix the problem reported by Alan in
https://lists.gnu.org/archive/html/emacs-devel/2019-10/msg01020.html
for the prompt asking about local variables:

  Please type y, n, or !, or C-v to scroll:

requires using read-char-from-minibuffer instead of read-char-choice.
A new arg CHARS was added to read-char-from-minibuffer to make it
a drop-in replacement for read-char-choice.

Here is a complete tested patch that solves all reported problems
in bug#17272 and bug#19064.  More specific patches will be posted
now to these bug reports as well.