[plt-scheme] 202.4

The exp-tagged code in CVS for MrEd and MzScheme is now version 202.4.

The only change is the addition of `open-input-output-file', which
takes the same arguments as `open-output-file', but returns two values:
an input port and an output port. The two ports use the same underlying
file handle/descriptor, but they have distinct (uncooperating) buffers.

This function is mostly useful for reading and writing to a device that
supports only a single open file handle/descriptor at a time.

Of course, many of you have asked for this function before. The recent
I/O rewrite made the addition possible, finally (though I didn't notice
until prompted by Mike S.).

Matthew