implement 'unit_base' option to select between KB and Kbit et. al.

implement 'unit_base' option to select between KB and Kbit et. al.

With network testing, it's often desirable to measure in terms of
kilobits/megabits rather than kilobytes/megabytes. This adds an option named
'unit_base' which can be set to either '1' or '8', where '1' means represent in
terms of bits and '8' means to represent rate in terms of bytes.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
Signed-off-by: Jens Axboe <axboe@kernel.dk>