fix is attached

test code:
(use-modules (ice-9 ftw))
(use-modules (ice-9 popen))
(define (print-open)
    (format #t "~a\n" (length (scandir "/proc/self/fd"))))
(let ((p (%make-void-port "rw")))
    (print-open)
    (parameterize ((current-output-port p) (current-input-port p) (current-error-port p))
      (system* "ls"))
    (close-port p)
    (print-open))
(parameterize ((current-error-port (%make-void-port OPEN_BOTH)))
      (while
       #t
       (close-pipe (open-pipe* OPEN_READ "free"))
       (print-open)
       (sleep 1)))

there's still a one-time increase in the file descriptors, but it seems unrelated. or at least much harder to find.

(by the way, ignoring current-output-port and using /dev/null is bad. it would be nice to open a pipe in that case and forward whatever the child writes to the scheme port.
but that would require a separate thread, and it may be impossible to do something like that for current-input-port)