system-hardening: added lynis

wiki/linux/system-hardening.md

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+# System Hardening
+
+System security is an important aspect of configuration.
+It will make it harder for attackers to get a hold of your system and private
+data.
+
+## Analysis Tools
+
+- [lynis](https://github.com/CISOfy/lynis): run `lynis audit system` to get a
+list of security flaws of your system