Flowgrind bulktrans

I extend flowgrind for bulk data transfer. Now you can transfer specific data size once using Z option.
Here is an example to transfer 10KB data in local network:
$ flowgrind -Z s=10000
The data transfer time appears as RTT in the final result.

This option cannot use traffic generation (-A, -G, -Z) and flow duration (-T) options. The usage message has been included in this branch.

I have an issue to be mentioned. Using Z option takes more time to generate flows than using G option when we generate more than 4 flows between two hosts. This issue occurs when we generate more than 4 flows, which are generated by different combination of hosts. The only way to avoid this problem is to use 10Gbps link for management link.
Why flow generation using Z option takes time is not clear yet. The only thing I know is to take longer time when flowgrind prepares flows (add_flow_source() in flowgrind.c). Using G option takes less than 4 millisecond in the loop of  this function when flowgrind generates 10 TCP connections. On the other hand,  using Z option takes more than 1 second. If you got the main cause, please apply your patch to this branch.

Also, I find a bug for flow parameter specification. It's fixed in this branch.