srfi-1 list-copy improper lists [1.6]

[Kevin Ryde]

Following up on similar in the head,

        * srfi-1.scm (list-copy): New function, allowing improper lists as per
        SRFI-1 spec, which the core list-copy doesn't do.

New code:

(define (list-copy lst)
  (let ((ret (cons #f lst)))
    (do ((lst lst (cdr lst))
         (end ret (cdr end)))
        ((not (pair? lst))
         (cdr ret))
      (set-cdr! end (cons (car lst) (cdr lst))))))