Portions of the code attempted to handle the fact that GPT entries on
disk may be larger than the currently defined struct while others
assumed the data could be indexed by the struct size directly. This
never came up because no utility uses a size larger than 128 bytes but
for the sake of safety we need to do this by the spec.
GPT_BOTH_VALID is 4 bits so simple a boolean check is not sufficient.
This broken condition allowed gptprio to trust bogus disk locations in
headers that were marked invalid causing arbitrary disk corruption.
Basic usage would look something like this:
gptprio.next -d usr_dev -u usr_uuid
linuxefi ($usr_dev)/boot/vmlinuz mount.usr=PARTUUID=$usr_uuid
After booting the system should set the 'successful' bit on the
partition that was used.
