[uaccess] Formalise the uaccess API

The userptr_t is now the fundamental type that gets used for conversions.
For example, virt_to_phys() is implemented in terms of virt_to_user() and
user_to_phys().

src/arch/i386/core/pic8259.c

@@ -16,6 +16,7 @@
  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
 
+#include <gpxe/io.h>
 #include <pic8259.h>
 
 /** @file