[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/registers.h

@@ -10,8 +10,7 @@
  *
  */
 
-#include "compiler.h" /* for doxygen */
-#include "stdint.h"
+#include <stdint.h>
 
 /**
  * A 16-bit general register.
@@ -184,4 +183,14 @@ struct i386_all_regs {
 #define SF ( 1 <<  7 )
 #define OF ( 1 << 11 )
 
+/* Segment:offset structure.  Note that the order within the structure
+ * is offset:segment.
+ */
+struct segoff {
+	uint16_t offset;
+	uint16_t segment;
+} PACKED;
+
+typedef struct segoff segoff_t;
+
 #endif /* REGISTERS_H */