bug#41045: transcoded-port turns input/output ports into output ports

Hello schemers,

The transcoded-port procedure from (rnrs) does not handle input/output
ports properly:

--8<---------------cut here---------------start------------->8---
GNU Guile 3.0.2
Copyright (C) 1995-2020 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
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Enter `,help' for help.
scheme@(guile-user)> (import (rnrs))
scheme@(guile-user)> (define p (open-file-input/output-port "/dev/null" (file-options no-fail)))
scheme@(guile-user)> (define tp (transcoded-port p (native-transcoder)))
scheme@(guile-user)> (values (output-port? tp) (input-port? tp))
$1 = #t
$2 = #f
--8<---------------cut here---------------end--------------->8---

Both return values should be #t.

-- 
Göran Weinholt   | https://weinholt.se/
Debian developer | 73 de SA6CJK