Ask user a yes-or-no question. Return t if answer is yes.
PROMPT is the string to display to ask the question. It should end in
a space; `yes-or-no-p' adds "(yes or no) " to it.
The user must confirm the answer with RET, and can edit it until it
has been confirmed.
Under a windowing system a dialog box will be used if `last-nonmenu-event'
is nil, and `use-dialog-box' is non-nil.
(fn PROMPT)