[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/boot/main.cpp

@@ -37,7 +37,7 @@ struct program_desc
 const program_desc program_list[] = {
 	{L"kernel", L"jsix.elf"},
 	{L"null driver", L"nulldrv.elf"},
-	{L"terminal driver", L"terminal.elf"},
+	//{L"terminal driver", L"terminal.elf"},
 };
 
 /// Change a pointer to point to the higher-half linear-offset version