systemd: make /etc/sysctl.conf have real effect

systemd: make /etc/sysctl.conf have real effect

In systemd, /etc/sysctl.conf is actually ignored by systemd-sysctl,
because this command only examine *.conf files under a bunch of directories
like /etc/sysctl.d or /usr/lib/sysctl.d.

The problem is we are used to configuring kernel parameters in /etc/sysctl.conf,
so it would be really strange if the configuration in that file doesn't have any
effect.

This patch reference Fedora's solution to this problem, creating a symlink to
/etc/sysctl.conf under /etc/sysctl.d/.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>