libfdisk: make it possible to use zero for size and start

libfdisk: make it possible to use zero for size and start

The zero may be valid size and start of the partition. This patch
introduces:

	fdisk_partition_has_start()
	fdisk_partition_has_size()
	fdisk_partition_unset_size()
	fdisk_partition_unset_start()

to make it possible to work with zero. The feature is internally
implemented by magic constant ((type) -1) for undefined sizes and
offsets.

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