[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/handle.h

@@ -1,30 +0,0 @@
-#pragma once
-/// \file handle.h
-/// Defines types for user handles to kernel objects
-
-#include "j6/types.h"
-#include "objects/kobject.h"
-
-class handle
-{
-public:
-	/// Move constructor. Takes ownership of the object from other.
-	handle(handle&& other);
-
-	/// Constructor.
-	/// \arg owner   koid of the process that has this handle
-	/// \arg rights  access rights this handle has over the object
-	/// \arg obj     the object held
-	handle(j6_koid_t owner, j6_rights_t rights, kobject *obj);
-
-	~handle();
-
-	handle() = delete;
-	handle(const handle &other) = delete;
-	handle & operator=(const handle& other) = delete;
-
-private:
-	j6_koid_t m_owner;
-	kobject *m_object;
-	j6_rights_t m_rights;
-};