Make TRACE write its messages in one go, to make it more
useful for debugging interactions between threads
GET-MUTEX now contains an assertion that we don't already
have the mutex
Don't make the failure of PTRACE_GETREGS kill the whole
instance. Not that thenew strategy is that much better
either, actually ...
Be slightly more careful about the order in which we remove
dead threads and do GC: i.e. do the GC afterwards.