more robust deadlock detection

more robust deadlock detection

 Lock around building the deadlock chain using WITH-CAS-LOCK after the
 tentative deadlock has been detected, and break the deadlock chain before
 signaling the error.

 This means that a single deadlock is reported only in a single thread.

 Fixes occasional failures of deadlock-detection.1 due to a bogus vicious
 metacircle. (Two threads detecting the same deadlock, then racing to report
 the error detected as another deadlock, the reporting of which in turn looked
 like a metacircle to CLOS if PRINT-OBJECT didn't yet have the right method in
 cache.)