[plt-scheme] file descriptor ports

Just wrote a C extension "make_fd_input_port" which wraps a POSIX file
descriptor as a scheme port.  While rummaging through the mzscheme
sources to see how I was supposed to handle the blocking stuff, I
noticed - as I should probably have guessed - that a more
comprehensive implementation of exactly this function already exists,
but it's not exposed to the scheme level.  I think it would be nice if
it were.

John