the '?' in read-multiple-choice prompt

[Shynur Xie]

The function `read-multiple-choice' always display a '?' in the
prompt.  E.g.,

    (read-multiple-choice "prompt" '((?a "alpha")))
    ;; prompt (alpha, ?):
                      ^
            How can I discard it?

Moreover, if evaluating

    (read-multiple-choice "prompt" '((?? "question")))
    ;; prompt (? question, ?):
               ^           ^
         How to distinguish them?