o PMBR partitions count to the number of partitions on the disk, which
means that if a PMBR entry is invalid we will not treat the MBR as a
PMBR by virtue of it not describing any partitions.
Previously the checks were inconsistent in that an invalid PMBR entry
would be harmless when no other partitions exist (we would treat the
MBR as a PMBR by virtue of it being empty), but it would be fatal when
there is at least one other partition.
o The partition size of a PMBR partition is one less than the media size
because the GPT starts at the second sector (LBA 1) and extends to
the end of the media. For backward bug-compatibility we accept a size
that's exactly the media size (FreeBSD bug).
Also, when the partition size can not be represented in a 32-bit
integral, the partition size in the MBR is to be set to 0xFFFFFFFF.
Accept this as a valid size, even if the size can be represented.
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