Tweak handling of configuration files

[bendem]

• Systematically add a / behind CONFIG_DIR. Previously, sometimes a / was added, sometimes not. Since it's common to write path without trailing slash, ensure it works in those cases.
• Don't try and modify the configuration file if it's already secure. We already setup our configuration so that only bastillion can read it and bastillion absolutely cannot modify its own configuration.

Let me know if you think these changes are appropriate.