It comes much more handy to compile in the IPv6 kernel module
(because it is loading almost everywhere) and disable the IPv6
functionality when the system starts up.
Therefore, IPv6 is not accidentially enabled at any time unless
someone wants to use it and disables the systcl options.
This commits removes some settings from /etc/sysctl.conf that have
been there forever with no particular reason.
They could improve performance on internet connections, especially
on lines with massive packet lost.
