libfdisk: (dos) allow to maximize partition

libfdisk: (dos) allow to maximize partition

The struct fdisk_partition has special flag "end_follow_default" to
make the partition large as much as possible. This patch makes this
flag usable for fdisk_set_partition() function.

Command line example (enlarge the first partition):

	# echo ',+' | ./sfdisk -N1 /dev/sdb
	...
	Disk /dev/sdb: 100 MiB, 104857600 bytes, 204800 sectors
	...
	Old situation:
	Device     Boot Start   End Sectors Size Id Type
	/dev/sdb1        2048 22527   20480  10M 83 Linux
			      ^^^^^
	New situation:
	Device     Boot Start    End Sectors Size Id Type
	/dev/sdb1        2048 204799  202752  99M 83 Linux
			      ^^^^^^

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