cfdisk: don't offer BSD

BSD is nested within DOS partition, so for disk without any partition
table it does not make any sense to offer BSD label.

Signed-off-by: Karel Zak <kzak@redhat.com>
cfdisk: add --zero command line option

The option has been supported by previous versions, we can easily
support it too.

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