handle-evt' cannot wrap handle-evt'

The prohbition against `handle-evt' on `handle-evt' is as
document and as originally intended. I'm not sure why it
was allowed.

Existing programs that use `handle-evt' incorrectly
can break. I found and fixed one incorrect use and one
questionable use in the Racket tree (which is a small
minority of the uses of `handle-evt' in the tree).

original commit: 084278fabcb3416250d7155faef674a31dac60b6

collects/mzlib/port.rkt

@@ -437,7 +437,7 @@
    (lambda (s)
      (let ([r (peek-bytes-avail!* s delta #f orig-in)])
        (set! delta (+ delta (if (number? r) r 1)))
-       (if (eq? r 0) (handle-evt orig-in (lambda (v) 0)) r)))
+       (if (eq? r 0) (wrap-evt orig-in (lambda (v) 0)) r)))
    (lambda (s skip default)
      (peek-bytes-avail!* s (+ delta skip) #f orig-in))
    void