bug#72092: popen creates "/dev/null" fds and never closes them

[jakub-w]

Consider the following example:

(use-modules (ice-9 popen))

(parameterize ((current-error-port (%make-void-port OPEN_BOTH)))
  (while #t
    (close-pipe (open-pipe* OPEN_READ "free"))
    (sleep 1)))

This opens a new "/dev/null" file descriptor every second and doesn't
close it when close-pipe is called.
If current ports are all fd ports, this doesn't happen.

AFAICT the problem was introduced by the commit
36fd2b4920ae926c79b936c29e739e71a6dff2bc in Guile 3.0.10.