kernel: msg: async messages no longer hold a pointer to the thread/port that sent them

this prevents a race condition where an event is sent as a port is being destroyed.
when the server gets around to handling the event, it now refers to a different port
that was created in the mean-time.

include/kernel/msg.h

@@ -21,8 +21,20 @@ struct msg {
 	enum kmsg_status msg_status;
 	struct btree_node msg_node;
 	msgid_t msg_id;
-	struct port *msg_sender_port;
-	struct thread *msg_sender_thread;
+	union {
+		/* only valid for asynchronous messages (msg_status ==
+		 * KMSG_ASYNC) */
+		struct {
+			koid_t msg_sender_port_id;
+			tid_t msg_sender_thread_id;
+		};
+		/* only valid for synchronous messages (msg_status !=
+		 * KMSG_ASYNC) */
+		struct {
+			struct port *msg_sender_port;
+			struct thread *msg_sender_thread;
+		};
+	};
 	kern_status_t msg_result;
 	kern_msg_type_t msg_type;