#24 corrupt backup set with afio

[Johan Bengtsson]

At least when backing up to files, perhaps also in other
modes can the backup set be corrupt during the dd
stage.

one parameter to dd is "ibs" setting the block size to
work with, this corrupts the output on my machine,
probably because incomplete blocks are read from the
output of afio. However changing this to "obs" instead
(output block size instead of input block size) solves the
problem and is probably what was intended in the first
place anyway.