A better solution to the funky filename problem

So, the problem was that our regexp for a Racket prompt didn't take
into account that filenames could contain white spaces: "@[^ ]*> ".  A
simple solution was accepting them: "@[^>]+> " won't work because '>'
is also a valid character in filenames, so we went for "@.*> ".

The drawback is that finding the beginning of the prompt (e.g. in C-a)
fails when you're writing things like:

   racket@foo bar.rkt> (> 2 3)

because here comint believes that the prompt is "racket@foo bar.rkt> (> "

And that could have side-effects elsewhere.  So what i've done is
simply changing the way white-space is (not) printed in the prompt,
substituting it by underscores.  That way, whe can go back to the
initial regexp, comint doesn't get confused, and users can easily
infer that "@foo_bar.rkt>" is actually referring to their
"foo bar.rkt" file.