examples: improve systemd unit files

Add the PrivateTmp, ProtectHome, and ProtectSystem directives to better
secure the system from chronyd. It's taken from the Debian chrony
package.

examples/chronyd.service

@@ -10,6 +10,9 @@ Type=forking
 PIDFile=/var/run/chronyd.pid
 EnvironmentFile=-/etc/sysconfig/chronyd
 ExecStart=/usr/sbin/chronyd $OPTIONS
+PrivateTmp=yes
+ProtectHome=yes
+ProtectSystem=full
 
 [Install]
 WantedBy=multi-user.target