free: implement --human

[cakebaker]

-h, --human
              Show all output fields automatically scaled to shortest three digit unit  and  dis-
              play the units of print out.  Following units are used.

                B = bytes
                Ki = kibibyte
                Mi = mebibyte
                Gi = gibibyte
                Ti = tebibyte
                Pi = pebibyte

              If  unit  is  missing,  and  you  have  exbibyte  of  RAM or swap, the number is in
              tebibytes and columns might not be aligned with header.

It's already partially implemented. What's missing is the "scaled to shortest three digit unit" functionality.