Changed cron_daily.php to avoid some php notices.

- Implemented FS#3308 - PATH placeholder in directive snippets (placeholder is named {DOCROOT}, like for nginx).

the path for the dkim-keys is automaticly creaed by the server-plugin, the path could be empty in the interface, and the amavis-template for gentoo was updated

Fixed issue with shell user creation due to missing parent user and group values in shell_user database records.

- Use it like $app->conf('yourpluginname', 'config-key', 'value')
- omit the value to read the config value, provide it to set the value

- database users
- email forwarders and aliases
- clients