1.0.46.42: more on threads in SB-INTROSPECT:MAP-ROOT

1.0.46.42: more on threads in SB-INTROSPECT:MAP-ROOT

 * Thread local values: filter out invalid values correctly (oops),
   also change SB-THREAD::%THREAD-LOCAL-REFERENCES to work on current
   thread only.

 * References from stack: SB-VM::MAP-STACK-REFERENCES walks the
   current thread stack looking for things that look like references
   to heap objects.

 * MAP-ROOT interrupts the thread it wants to check out, unless it is
   the current one -- a bit tacky, but seems preferable to stopping
   the world.