[kernel] Let endpoints get interrupt notifications

- Add a tag field to all endpoint messages, which doubles as a
  notification field
- Add a endpoint_bind_irq syscall to enable an endpoint to listen for
  interrupt notifications. This mechanism needs to change.
- Add a temporary copy of the serial port code to nulldrv, and let it
  take responsibility for COM2

src/kernel/objects/endpoint.h

@@ -26,23 +26,33 @@ public:
 
 	/// Send a message to a thread waiting to receive on this endpoint. If no threads
 	/// are currently trying to receive, block the current thread.
+	/// \arg tag   The application-specified message tag
 	/// \arg len   The size in bytes of the message
 	/// \arg data  The message data
 	/// \returns   j6_status_ok on success
-	j6_status_t send(size_t len, void *data);
+	j6_status_t send(j6_tag_t tag, size_t len, void *data);
 
 	/// Receive a message from a thread waiting to send on this endpoint. If no threads
 	/// are currently trying to send, block the current thread.
+	/// \arg tag   [in] The sender-specified message tag
 	/// \arg len   [in] The size in bytes of the buffer [out] Number of bytes in the message
 	/// \arg data  Buffer for copying message data into
 	/// \returns   j6_status_ok on success
-	j6_status_t receive(size_t *len, void *data);
+	j6_status_t receive(j6_tag_t *tag, size_t *len, void *data);
+
+	/// Give the listener on the endpoint a message that a bound IRQ has been signalled
+	/// \arg irq  The IRQ that caused this signal
+	void signal_irq(unsigned irq);
 
 private:
 	struct thread_data
 	{
 		thread *th;
 		void *data;
+		union {
+			j6_tag_t *tag_p;
+			j6_tag_t tag;
+		};
 		union {
 			size_t *len_p;
 			size_t len;