#6 file sink mode

[Jon Dugan]

add a file sink mode, so that along with the -F filename option,
one can also do true disk-2-disk testing.
measure disk throughput as well as network throughput on both ends
when doing disk2disk testing.
[not sure this is within the scope of Iperf, but maybe -jdugan]

From Brian Tierney LBL/ESnet