Custom default Linux sysctl settings

This doesn't include any automatic support for installing or using these
files.  Most Linux distros [verify] will automatically apply any `.conf`
file in /etc/sysctl.d, so you probably want something like:

    $ dotfiles install
    $ ln -s $HOME/.config/linux-defaults.conf /etc/sysctl.d
    $ sudo sysctl -p /etc/sysctl.d/linux-defaults.conf

(The final command is only needed once, to pick up the settings in the
new file without requiring a reboot.)

From http://tobert.github.io/post/2014-06-24-linux-defaults.html