Enhance transcoder-related functionality of `(rnrs io ports)'

Enhance transcoder-related functionality of `(rnrs io ports)'

* module/rnrs/io/ports.scm (transcoder-eol-style)
  (transcoder-error-handling-mode): Export these.
  (textual-port?): Implement this procedure and export it.
* module/rnrs.scm: Export these here as well.

* module/rnrs/io/ports.scm (port-transcoder): Implement this procedure.
  (binary-port?): Treat only ports without an encoding as binary ports,
  add docstring.
  (standard-input-port, standard-output-port, standard-error-port):
  Ensure these are created without an encoding.
  (eol-style): Add `none' as enumeration member.
  (native-eol-style): Switch to `none' from `lf'.

* test-suite/tests/r6rs-ports.test (7.2.7 Input ports)
  (8.2.10 Output ports): Test binary-ness of `standard-input-port',
  `standard-output-port' and `standard-error-port'.
  (8.2.6 Input and output ports): Add test for `port-transcoder'.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>