fdisk: add --wipe

This patch changes fdisk behavior and it wipes foreign signatures
from the device to avoid collisions. The wipe functionality is
automatically enabled in the interactive mode only (user is always
warned about it), otherwise it's possible to control all by --wipe
<auto|never|always>.

The program does not change behavior when executed in scripts (echo
<something> | fdisk), the option "--wipe=always" is required to enable
in this case.

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