Finally get rid of the eval() in the captcha code
    
    I've hated this forever, and I finally decided to get rid of it
    after writing some intemperate comments about how using eval()
    is awful and no-one should ever do it.
    
    This still doesn't actually *work* any more, but it fails to
    work just the same way as the old code, so it's no worse! I
    don't recall all the details of why it doesn't work, but it's
    something like like mediawiki doesn't recognize that we're the
    'same' session that it sent the question to, so it doesn't care
    that we got the answer right, it just starts over with another
    question. I may dig into this a bit more with puiterwijk and
    fix it tomorrow, but we've been relying on just not hitting this
    code for months.