Don't do anything when running zkg autoconfig on a Zeek-bundled install

[ckreibich]

We keep seeing that people run zkg autoconfig on the standard Zeek-bundled zkg setup. Permission issues aside this is at least confusing and at worst introduces brittleness because the config does get overwritten. zkg should recognize this setting, indicate that there's no need to do anything, and exit.

When the user provides a custom config or --user, autoconfig continues to make sense.