tcp: add a sysctl to config the tcp_default_init_rwnd

The default initial rwnd was 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: I7eac8a0a5133371aea9ecb9aec0b608bd7f2cc57
Signed-off-by: JP Abgrall <jpa@google.com>
Disable CONFIG_STRICT_MEMORY_RWX

Its intention is to protect against theoretical vulnerabilities, but
in reality it opens the door for an already actively used exploit.
https://www.codeaurora.org/projects/security-advisories/configstrictmemoryrwx-not-strictly-enforced-cve-2013-4737
The security patch that fixes CONFIG_STRICT_MEMORY_RWX is expensive.
Better conserve the resources and protect us by simple disable of it.

Change-Id: I2a27722efa8294f0aa22d864d5fb96e8f003d3f0