[plt-scheme] MysterX with vs.NET

Last week, I changed the Windows overnight builds to use Visual Studio
.NET.

While it compiled fine, MysterX did not run fine.  In particular, the
demo program crashed.

I've updated a few source files and rebuilt binaries, now available on
the download server.

I've decided to bite the bullet and require vs.NET for builds.  We
supply binaries, so that should not be an issue unless you want to play
with the sources.  In that rare case, you will need vs.NET.

-- Paul