Add page for firewall-cmd in linux

pages/linux/firewall-cmd.md

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+# firewall-cmd
+
+> The firewalld command line client
+
+- View the available firewall zones
+
+`firewall-cmd --get-active-zones`
+
+- View the rules which are currently applied
+
+`firewall-cmd --list-all`
+
+- Permanently open the port for a service in the specified zone (like port `443` when in the `public` zone)
+
+`firewall-cmd --permanent --zone={{public}} --add-service={{https}}`
+
+- Permanently close the port for a service in the specified zone (like port `80` when in the `public` zone)
+
+`firewall-cmd --permanent --zone={{public}} --remove-service={{http}}`
+
+- Reload firewalld to force rule changes to take effect
+
+`firewall-cmd --reload`