Back off a couple of the more paranoid checks while spoofing GPT

Back off a couple of the more paranoid checks while spoofing GPT
partitions into the disklabel.

First, since the alt header is never accessed there is no need to
worry about it being inaccessible.

Second, the GPT header claiming to cover more sectors than the
device has is no reason to ignore all the partitions. The
partition actually present could still be useful.

Issues encountered in the wild by mlarkin@ while accessing some
disk images.

ok deraadt@