- Fix the case where we're not preempting but there is already a newtd

 - Fix the case where we're not preempting but there is already a newtd
   as this happens via thread_switchout().  I don't particularly like the
   structure of the code here.  We twice call out to thread code when
   a thread is voluntarily switching.  Once to thread_switchout() and once
   to slot_fill(), while sched_4BSD does even more work which is redundant
   to select another thread to use our remaining slice.  This should be
   simplified in the future, but for now I'm only going to fix the bug not
   the bad design.