libfdisk: Add support for altering GPT size

libfdisk: Add support for altering GPT size

This is useful in two situations:

1. More than 128 partitions are required. Or

2. The partition table must be restricted in size, such as when a system
expects to find a bootloader at a location that would otherwise overlap the
partition table.

The gdisk partitioner supports this feature.

libfdisk is already capable of reading and writing partition tables of any
size, but previously could only create ones of 128 entries and could not
resize.

This change should be fairly safe, as it has no effect unless explicitly
activated.

Signed-off-by: Karel Zak <kzak@redhat.com>