[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/drivers/nulldrv/serial.h

@@ -0,0 +1,26 @@
+#pragma once
+/// \file serial.h
+/// Declarations related to serial ports.
+#include <stdint.h>
+
+#define serial_port nulldrv_serial_port
+
+class serial_port
+{
+public:
+	/// Constructor.
+	/// \arg port   The IO address of the serial port
+	serial_port(uint16_t port);
+
+	serial_port();
+
+	void write(char c);
+	char read();
+
+private:
+	uint16_t m_port;
+
+	bool read_ready();
+	bool write_ready();
+};
+