Fixed (partly) FS#2310 - Template switch not working in mailuser module, for dashlets and the login page

Merged revisions 3049-3051,3053-3054,3058,3070,3074-3077,3082,3086-3088,3091-3092,3094-3095 from stable branch.

- nginx: merge function: default action is now to replace older locations in the nginx vhost configuration; a merge  instead of a replacement can be achieved by putting the string "##merge##" (without quotes) at the end of the most recent location line (e.g. "location ~ \.php$ { ##merge##").

- Installer adds Apache and nginx user to ispapps group.
- Installer makes sure that /var/www/apps has 755 permissions.

- 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.

- Changed FastCGI buffers for ISPConfig nginx vhost because otherwise you get "upstream sent too big header while reading response header from upstream" on OpenSUSE.
- Added 502 error page ("Bad Gateway").
- Added "fastcgi_intercept_errors on;" to nginx vhost template so that ISPConfig's error pages are displayed instead of the default nginx ones.

- 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.

- Added "ipv6only=on" to the IPv6 listen lines in the nginx vhost template to avoid "[emerg]: bind() to [::]:80 failed (98: Address already in use)" errors.

- Added SNI config field in sysetm > server config > web.

Implemented: FS#1113 - Force client to use only allowed IPs