* removes --vex-guest-chase-cond=no|yes. This was never used in practice.
* rename --vex-guest-chase-thresh=<0..99> to --vex-guest-chase=no|yes. In
otherwords, downgrade it from a numeric flag to a boolean one, that can
simply disable all chasing if required. (Some tools, notably Callgrind,
force-disable block chasing, so this functionality at least needs to be
retained).