libfdisk: add API to control signatures wiping

libfdisk: add API to control signatures wiping

Now libfdisk warns if another filesystem/RAID/PT signature is detected
on the device. It seems better and user-friendly to make it possible to
wipe the signatures when we write a new disk label to the device.

This patch adds to the library API

	fdisk_enable_wipe()
	fdisk_has_wipe()
	fdisk_get_collision()

to control this new feature. The device modification is done by
libblkid (the same we use in some mkfs-like utils).

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