[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/gdbidt.S

@@ -2,7 +2,7 @@
  * Interrupt Descriptor Table (IDT) setup and interrupt handlers for GDB stub.
  */
 
-#include <virtaddr.h>
+#include <librm.h>
 
 #define SIZEOF_I386_REGS	32
 #define SIZEOF_I386_FLAGS	4