Fixed: FS#3226 - mail_user_update FAIL for changes with quota >2048 MB (Soap Error: quota_error_isint)

- 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