sfdisk: add --discard-free

sfdisk: add --discard-free

Why do we need this? It can be difficult for end-users to discard
unpartitioned areas using blkdiscard, as it requires using fdisk to
obtain a list of free areas and then using blkdiscard with the correct
--offset and --length options. It is less risky for end-users to use
(s)fdisk, as they have a better understanding of the disk layout.

Addresses: https://github.com/util-linux/util-linux/issues/2804
Signed-off-by: Karel Zak <kzak@redhat.com>