[project @ 1999-01-20 16:07:40 by simonm]

The BLACKHOLEs created when entering a CAF for the first time are now
CAF_BLACKHOLES, distinguishing them from the blackholes created by
lazy blackholing.

This enables the lazy blackholing algorithm to be optimised by not
blackholing a section of stack that has already been traversed.