Add initial HIP_SYNC_NULL_STREAM=0 mode.

This eliminates host-synchronization for null stream.  Instead, the
null-stream uses GPU-side events to wait for other streams.
Default is OFF pending additional testing.

Add enhanced null-stream test.

Also refine HIP_TRACE_API.


[ROCm/hip commit: 27877f8854]

projects/hip/include/hip/hcc_detail/hip_runtime_api.h

@@ -602,9 +602,12 @@ hipError_t hipStreamQuery(hipStream_t stream);
  *
  * @return #hipSuccess, #hipErrorInvalidResourceHandle
  *
- * If the null stream is specified, this command blocks until all
+ * This command is host-synchronous : the host will block until the specified stream is empty.
+ *
+ * This command follows standard null-stream semantics.  Specifically, specifying the null stream will cause the 
+ * command to wait for other streams on the same device to complete all pending operations.
+ *
  * This command honors the hipDeviceLaunchBlocking flag, which controls whether the wait is active or blocking.
- * This command is host-synchronous : the host will block until the stream is empty.
  *
  * @see hipStreamCreate, hipStreamCreateWithFlags, hipStreamWaitEvent, hipStreamDestroy
  *