Changed cron_daily.php to avoid some php notices.

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

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

- Added email message templates.

- Fill IP select fields with IP from correct server even if the user's default web server is another server. Otherwise if the user opens the web form, the wildcard * is selected, and if he changes the tab, the wildcard is saved instead of the correct IP so that the website will no longer work.

- nginx pm settings: values were limited to 3 figures (so max. value was 999), increased this to 6 figures.

- Added section toggle for client limits

- Resellers can use templates for their clients.