y-or-n-p actually a conspiracy involving SPC and DEL too

[Dan Jacobson]

  y-or-n-p is a built-in function.

  Ask user a "y or n" question.  Return t if answer is "y".
  Takes one argument, which is the string to display to ask the question.
  It should end in a space; `y-or-n-p' adds `(y or n) ' to it.
  No confirmation of the answer is requested; a single character is enough.
  Also accepts Space to mean yes, or Delete to mean no.  (Actually, it uses

well, as a matter of trust between the user and emacs, you might
mention "(y,SPC or n,DEL)".

That way the user will have some clue as to why all is hunky dory
after he hits one of the choices not listed.

And when one hits a key beyond the four, do ask for one of the four
and not just mentioning the two.
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