*** y-or-n-p-with-timeout lets you ask a question but give up if<BR>
there is no answer within a certain time.
<P>
  (y-or-n-p-with-timeout PROMPT SECONDS DEFAULT-VALUE)
<P>
asks the question PROMPT (just like y-or-n-p).  If the user answers
within SECONDS seconds, it returns the answer that the user gave.
Otherwise it gives up after SECONDS seconds, and returns DEFAULT-VALUE.
<P>