changed "fmods/s" to "flows/s" in output

to better reflect that we're also counting packet outs
Added dpid-offset parameter

When running more than one cbench instance targetted at the same
controller, DPIDs would overlap, which causes some controllers
to error out.  This new parameter allows users to select the
initial offset of the emulated switchs' DPIDs.
replaced a few literal '65536' with 'BUFLEN'

silly people filling the code with literals
Merge git://gitosis.stanford.edu/oflops