Fix performance on 10Gb networks

[Iuliu Rus]

Hello,
Google is running Cygwin apps on its 10Gb networks and we are seeing
extremely bad performance in a couple of cases. For example, iperf
with the defaults results in only 10Mbits/sec.
We tracked this down to a combination of non-blocking sockets with
Nagle+delayed ack kicking in, since the apps eventually end up sending
a very small packets (2 bytes).
We have a case open against Microsoft but since everything is moving
very slow we would like to work around by picking socket buffers that
are multiple of 4k.

Change log:
2014-11-18 Iuliu Rus <rus.iuliu@gmail.com>

* net.cc Change default values for socket buffers to fix performance
on 10Gb networks.
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