[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/firmware/pcbios/gateA20.c

@@ -1,6 +1,7 @@
 #include <stdio.h>
 #include <realmode.h>
 #include <bios.h>
+#include <gpxe/io.h>
 #include <gpxe/timer.h>
 
 #define K_RDWR		0x60		/* keyboard data & cmds (read/write) */