gptmbr: implement the new T13-approved GPT protocol

gptmbr: implement the new T13-approved GPT protocol

My GPT-based protocol was modified by the UEFI and T13 committees (the
former responsible for the GPT format, the latter for EDD and
therefore for the disk-related part of the BIOS specification.)  This
is thus now on its way to become an official protocol, so change the
implementation to match.

This still needs testing, and the Syslinux core needs to be adjusted
to leverage the extended information.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>