profiler: Actually support profiling user stack frames only.

The "-k" argument (which never did anything before) is now inverted
compared to what it used to be, i.e. now specifying it will profile
kernel frames, too, whereas by default only user frames will be
sampled.

headers/private/system/system_profiler_defs.h

@@ -21,6 +21,7 @@ struct system_profiler_parameters {
 	// sampling
 	bigtime_t	interval;				// interval at which to take samples
 	uint32		stack_depth;			// maximum stack depth to sample
+	bool		profile_kernel;			// sample kernel stack frames
 };