[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/include/int13.h

@@ -9,6 +9,7 @@
 
 #include <stdint.h>
 #include <gpxe/list.h>
+#include <realmode.h>
 
 struct block_device;