fdisk: move "create label" actions to menu callback

Signed-off-by: Karel Zak <kzak@redhat.com>
fdisk: cleanup sgi menu callbackswq

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: cleanup SGI functions names

Signed-off-by: Karel Zak <kzak@redhat.com>
fdisk: consolidate code in fdisk.c

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