Indicate lock status in CONDITION-WAIT return.

On SBCL, BT:CONDITION-WAIT is identical to SB-THREAD:CONDITION-WAIT,
returning T if the lock is held and NIL otherwise.

On non-SBCL implementations, BT:CONDITION-WAIT always returns T,
as these implementations reacquire the lock before returning.
Merge Github pull request #5: Indicate lock status in CONDITION-WAIT return