losetup: add --list and --output option

losetup: add --list and --output option

The --list --all is the default output:

  # losetup
  NAME       SIZE OFFSET AUTOCLEAR RO BACK-FILE
  /dev/loop0    0      0         0  0 /home/fs-images/swap.img
  /dev/loop1    0    100         0  0 /home/fs-images/disk.img

[kzak@redhat.com: - default to --all if no device specified,
                  - cleanup column names (add BACK- prefix)
                  - add MAJ:MIN to of the loopdev device]

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