[kernel] Give processes and threads self handles

It was not consistent how processes got handles to themselves or their
threads, ending up with double entries. Now make such handles automatic
and expose them with new self_handle() methods.

src/kernel/objects/process.h

@@ -68,6 +68,9 @@ public:
 	/// \returns  True if this thread ending has ended the process
 	bool thread_exited(thread *th);
 
+	/// Get the handle for this process to refer to itself
+	inline j6_handle_t self_handle() const { return 0; }
+
 	/// Get the process object that owns kernel threads and the
 	/// kernel address space
 	static process & kernel_process();