[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/config/defaults/pcbios.h

@@ -7,6 +7,7 @@
  *
  */
 
+#define UACCESS_LIBRM
 #define IOAPI_X86
 #define PCIAPI_PCBIOS
 #define TIMER_PCBIOS