Update server/conf/nginx_apps.vhost.master, server/conf/nginx_vhost.conf.master, server/conf/vhost.conf.master files

ISPConfig itself does not read nor interpret this header, but there might be other
server components like PHP that might interact with it so I'll disable it just to be sure.

Replace fastcgi_pass unix:/var/run/php5-fpm.sock; with fastcgi_pass unix:{fpm_socket}; in phpmyadmin and squirrelmail blocks, as they are considered apps and should use the same (apps.sock) listener.

- Added check for the apps vhost to find out if the default PHP-FPM pool uses port 9000 or the socket /var/run/php5-fpm.sock.

- 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.