[R6RS] `open-string-output-port' is the same as `open-bytes-output-port'

[dyb at cs.indiana.edu]

> Just a note to make sure everybody's aware of that.

This probably makes perfect sense, but can you please explain briefly why
this is (and should be) so?  Also, why does it take a transcoder argument? 
For that matter, why do the open-file-input-port and open-file-output-port
procedures take a transcoder argument?  I thought we'd decided to
eliminate it.

Kent