Add display of MB/s.

Use HiRes time() only for the total rate, makes no sense
to use it in the loop and keep int():ing it, and output
the epoch timestamp.

Default the file to out.srl.
Allow --count=-1 to mean 'forever'.
Make ^C:ing to show the totals.
Allow sending a certain number of megabytes.
Stem the flood of variables with %Opt.
Allow running for N seconds.
The number of packets is integral.
Better label for the data size.
Fix loop MB/s message.
Better usage and failure messages.
Label the epoch.
Log the packet/sec rate in the test listener.