srfi-39 parameters current-input-port

[Kevin Ryde]

The srfi-39 spec calls for current-input-port and current-output-port
to be implemented as parameter objects.  That will mean new #:replace
bindings exported I think.

There's no actual fluid underlying current-input-port is there?  Could
that be done, so that with-fluids* can be used like on other parameter
objects?  Or should parameterize and with-parameters* do an explicit
dynamic-wind?

(current-error-port could get the same replacement too, for
consistency.)