[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/thread.h

@@ -141,6 +141,9 @@ public:
 	/// \arg rip  The address to return to, must be user space
 	void add_thunk_user(uintptr_t rip);
 
+	/// Get the handle representing this thread to its process
+	j6_handle_t self_handle() const { return m_self_handle; }
+
 	/// Create the kernel idle thread
 	/// \arg kernel The process object that owns kernel tasks
 	/// \arg pri    The idle thread priority value
@@ -175,4 +178,6 @@ private:
 	uint64_t m_wait_data;
 	j6_status_t m_wait_result;
 	j6_koid_t m_wait_obj;
+
+	j6_handle_t m_self_handle;
 };