Limit available PHP Handlers globally in ISPConfig UI

ISPConfig still supports several old PHP handlers for legacy reasons (suPHP, CGI and HHVM), but they should not be available on new systems. A new option has been added under System > Interface > Main config to hide old PHP handlers in the UI. The new default is to show only php-fcgi, php-fpm and mod_php. This option is relevant for Apache servers only as Nginx systems use just php-fpm anyway.