sfdisk: add --backup and --backup-file

sfdisk: add --backup and --backup-file

The option --backup force sfdisk to store *all* fragments of the
partition table (including MBR partition tables store in the
extended partitions) to

	$HOME/sfdisk-<devname>-<offset>.bak

The options -O, -backup-file <path> allows to override the default
path, but sfdisk still appends <devname>-<offset>.bak to the <path>.
The backup files always contain only raw data from the device, so it's
possible to use dd(1) to restore original data on the device.

The original sfdisk also supported -O <file>, but semantic was little
bit different:

   - all was based on 512-byte sectors
   - all sectors was stored to the one file in format
	  <offset>|<sector>|<offset>|...

this original concept makes the backup files specific to sfdisk and with
dependence on sector size.

The new concept is the same we already use for wipefs(8) backup files.

Example (disk with GPT):

   # sfdisk /dev/sda --backup

   Welcome to sfdisk (util-linux 2.25.202-f4deb-dirty).
   Changes will remain in memory only, until you decide to write them.
   Be careful before using the write command.

   Backup files:
          PMBR (offset     0, size   512): /root/sfdisk-sda-0x00000000.bak
    GPT Header (offset   512, size   512): /root/sfdisk-sda-0x00000200.bak
   GPT Entries (offset  1024, size 16384): /root/sfdisk-sda-0x00000400.bak

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