bug#39610: R6RS `flush-output-port` not playing along with `transcoded-port`

Hi fellow Schemers!

I was gently nudged on IRC into having a look into my Scheme code, after
many years of abandon, and found that `dorodango` (a reasonably large
body of R6RS code) hangs with Guile 2.2 and 3.0, while it worked on 2.0
(IIRC). I isolated the cause; the following snippet hangs on Guile 2.2
and 3.0, while it worked as expected on 2.0:

;; ------------------
(import (rnrs))

(let* ((p (pipe))
       (in (car p))
       (out (transcoded-port (cdr p) (make-transcoder (utf-8-codec)))))
  (put-datum out "foo")
  (flush-output-port out)
  (display "Should have written to pipe by now, attempting reading a byte\n")
  (display "Got")
  (display (get-u8 in))
  (newline))
;; -------------------

It seems the underlying port is no longer flushed to the OS, so the
`get-u8` now hangs waiting for input, starting with Guile 2.2.

Regards, Rotty