This patch adds makes TCP use a larger initial window (slow-start)
for hosts on a local network.  There seems to be little worry of
congesting a router when on a local net :-)

This gains about 10% in small-socket-write (1K and 2K) TCP benchmark
performance for local net hosts on an ARM platform that Allen Briggs
and I are working with.

FWIW, FreeBSD also uses a larger initial window for local networks, so
we're clearly not the only ones which have seen a benefit from this :-)

Comments?  I'm particularly interested in how the inet vs inet6 stuff
is handled (man, the way we do that is icky).

-- 
        -- Jason R. Thorpe <thorpej@wasabisystems.com>