Add non-randomize option to pspgen.

 - If you want to generate a fixed number of flows, you use `-f` option
   as before, but can add `-r 0` option (default is 1) to make the
   destination addresses to increase one-by-one instead of take random
   numbers.
   (e.g., `-f 100 -r 0` option assigns 10.0.0.1 to 10.0.0.100
   destination addresses to 100 flows)