Turn on systemd ProtectSystem

This will protect various files from being edited by webdavd.

Additional protections, e.g. ProtectHome, cause a problem with webdavd's
worker attempts to start a new session as the remote user.

package-with/systemd.service

@@ -7,6 +7,7 @@ After=network.target
 Type=simple
 ExecStart=/usr/sbin/webdavd /etc/webdavd
 LogsDirectory=webdavd
+ProtectSystem=full
 
 [Install]
 WantedBy=multi-user.target