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/os/kernel/debugger.h

@@ -407,6 +407,7 @@ typedef struct {
 	bool				variable_stack_depth;
 										// variable number of samples per hit;
 										// cf. debug_profiler_update
+	bool				profile_kernel;	// sample kernel stack frames
 } debug_nub_start_profiler;
 
 typedef struct {