add support for negative authorizations

add support for negative authorizations

Negative authorizations is a way to block an entity; previously the
algorithm was something like (ignoring the config file for now)

  Result is_authorized() {
    res = has_implicit_auth();
    if (res == YES) {
      return YES;
    } else if (has_explicit_auth()) {
      return YES;
    }
    return res;
  }

Now it's

  Result is_authorized() {
    res = has_implicit_auth();
    expl = has_explicit_auth();
    is_blocked = has_negative_explicit_auth();

    if (is_blocked)
      return NO;

    if (res == YES) {
      return YES;
    } else if (has_explicit_auth()) {
      return YES;
    }
    return res;
  }

E.g. just a single negative auth will force NO to be returned. I
really, really need to write into the spec how this works; my mental
L1 cache can't contain it anymore. Once it's formally defined we need
to craft a test suite to verify that the code works according to
spec...