[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/drivers/net/undinet.c

@@ -23,6 +23,7 @@
 #include <biosint.h>
 #include <pnpbios.h>
 #include <basemem_packet.h>
+#include <gpxe/io.h>
 #include <gpxe/iobuf.h>
 #include <gpxe/netdevice.h>
 #include <gpxe/if_ether.h>