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/kernel/user_debugger.h

@@ -147,6 +147,8 @@ struct thread_debug_info {
 			// the buffer
 		bool			variable_stack_depth;
 			// record a variable number of samples per hit
+		bool			profile_kernel;
+			// record samples in kernel stack frames
 		bool			buffer_full;
 			// indicates that the sample buffer is full
 		union {