adds the hooks that DTrace modules register with, and adds a few functions which have the dtrace_ prefix to allow the DTrace FBT (function boundary trace) provider to avoid tracing because they are called from the DTtrace probe context. Unlike other forms of tracing and debug, DTrace support in the kernel incurs negligible run-time cost. I think the only reason why anyone wouldn't want to have kernel support enabled for DTrace would be due to the license (CDDL) under which DTrace is released.