[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/io.cpp

@@ -0,0 +1,22 @@
+#include "io.h"
+
+uint8_t
+inb(uint16_t port)
+{
+	uint8_t val;
+	__asm__ __volatile__ ( "inb %1, %0" : "=a"(val) : "Nd"(port) );
+	return val;
+}
+
+void
+outb(uint16_t port, uint8_t val)
+{
+	__asm__ __volatile__ ( "outb %0, %1" :: "a"(val), "Nd"(port) );
+}
+
+void
+io_wait(unsigned times)
+{
+	for (unsigned i = 0; i < times; ++i)
+		outb(0x80, 0);
+}