partx: support loop devices

partx: support loop devices

Add support for loop devices to add partitions. For now we make use of the
max_part parameter from the loop kernel module, otherwise the feature is
disabled.

Below an example output:

root@offbook:~/projects/util-linux/partx# ./partx -a -n 1:5 images-pt/dos+bsd.img
root@offbook:~/projects/util-linux/partx# ls /dev/loop0* -ltr
brw-rw---- 1 root disk 7, 0 2011-08-15 00:07 /dev/loop0
brw-rw---- 1 root disk 7, 5 2011-08-15 00:07 /dev/loop0p5
brw-rw---- 1 root disk 7, 2 2011-08-15 00:07 /dev/loop0p2
brw-rw---- 1 root disk 7, 1 2011-08-15 00:07 /dev/loop0p1

Signed-off-by: Davidlohr Bueso <dave@gnu.org>
Signed-off-by: Karel Zak <kzak@redhat.com>