[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/main.cpp

@@ -17,7 +17,6 @@
 #include "log.h"
 #include "objects/channel.h"
 #include "objects/event.h"
-#include "objects/handle.h"
 #include "scheduler.h"
 #include "serial.h"
 #include "symbol_table.h"