set the ddclient.conf permissions to 600

An issue #1 (closed) was opened because ddclient complains if the permissions on the config are too accessible. The comment in the code suggests the file should be owned by root:ddclient and have permissions 640.

However the code has the requirements that the file is only accessible by its owner, and that the owner is the EUID.

The shipped systemd file runs the service as root, meaning there's no point in creating a ddclient group as it will have no access to the file anyway. The file cannot be owned by a ddclient user without modifying the systemd service.

fixes #1 (closed)