Set default env[PATH] and env[HOSTNAME] since it is cleared by default setting clear_env = yes in php-fpm

- Enable php-fpm socket mode as default.

- Added PHP-FPM pm settings (pm.max_children, pm.start_servers, pm.min_spare_servers, pm.max_spare_servers) to the "Options" tab of a website.

- Installer adds ispconfig user to Apache group on Fedora/CentOS so that nginx/PHP-FPM can access /etc/squirrelmail and serve SquirrelMail for the ISPConfig vhost.
- Added session.save_path and upload_tmp_dir settings to the PHP-FPM pool definitions.

- Enabled Security Level (Server Config > Web > Security Level) for nginx.