1.0.37.8: add ATOMIC-DECF, fix WAIT-ON-SEMAPHORE-BUGLET

1.0.37.8: add ATOMIC-DECF, fix WAIT-ON-SEMAPHORE-BUGLET

 * We already had SB-EXT:ATOMIC-INCF, so this seems a sensible
   companion. I really cannot remember why I didn't do things like
   this in the first place -- lack of time, maybe?

 * Use ATOMIC-DECF instead of DECF in WAIT-ON-SEMAPHORE to decrement
   the waitcount: if we unwind from CONDITION-WAIT due to timeout the
   mutex might no longer be ours.