tcp: add a sysctl to config the tcp_default_init_rwnd [Abgrall]

The default initial rwnd is hardcoded to 10.

Now we allow it to be controlled via
 /proc/sys/net/ipv4/tcp_default_init_rwnd
 which limits the values from 3 to 100

This is somewhat needed because ipv6 routes are
 autoconfigured by the kernel.

See "An Argument for Increasing TCP's Initial Congestion Window"
 in https://developers.google.com/speed/articles/tcp_initcwnd_paper.pdf

Change-Id: I386b2a9d62de0ebe05c1ebe1b4bd91b314af5c54

Signed-off-by: JP Abgrall <jpa@google.com>