I don't think this is urgently crucial but it is a good improvement to
the relevant code.

The main change is to protect the running of the unwind blocks so they
won't fire twice - though so far as I can see this only has any effect
on unwind blocks involved in exception handling, of which there seems to
be a small number. The loop to detect unwind blocks is made more
intelligable although by fluke of the way terminateTo: works the
fuctionality remains the same.