[ns] adding TCP Rate-Halving to NS.

[Sally Floyd <floyd@aciri.org>]

I have added the TCP Rate-Halving code from the contributed code
web page to the NS distribution.  The TCP Rate-Halving code is
from Matt Mathis, Jeff Semke, Jamshid Mahdavi, and Kevin Lahey, 
from the web page
``http://www.psc.edu/networking/ftp/papers/draft-ratehalving.txt''.
This is partially validated in the validation test
"./test-all-tcpVariants", for example in
"./test-all-tcpVariants/fourdrops_sackRH".

- Sally
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http://www.aciri.org/floyd/
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