gpt: create empty disklabels

gpt: create empty disklabels

This patch enables creating a new, empty, GPT disklabel from either
an empty disk or one that already has a disklabel. For this
purpose, a 'g' option is added to the main menu and is visible to all
labels. Here's an example for a scsi_debug device (/dev/sdb):

...
Device does not contain a recognized partition table
Building a new DOS disklabel with disk identifier 0x20a614c8.
3696: fdisk:  CONTEXT: zeroize in-memory first sector buffer

Command (m for help): g
3696: fdisk:    LABEL: changing to gpt label

3696: fdisk:  CONTEXT: zeroize in-memory first sector buffer
3696: fdisk:    LABEL: created new empty GPT disklabel (GUID: D4EA0706-F011-46DC-B7DE-6A72C7090AF8)

Command (m for help): w
The partition table has been altered!
...

Acked-by: Petr Uzel <petr.uzel@suse.cz>
Signed-off-by: Davidlohr Bueso <dave@gnu.org>