sysctl.linux.in: use the --system option

According to the sysctl man page, the --system option causes sysctl to
process all system configuration files, which include the following:

/run/sysctl.d/*.conf
/etc/sysctl.d/*.conf
/usr/local/lib/sysctl.d/*.conf
/usr/lib/sysctl.d/*.conf
/lib/sysctl.d/*.conf
/etc/sysctl.conf

X-Gentoo-Bug: 484796
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=484796