Fix nginx admin_menu problem (with cache setting?)

There is a weird interaction between nginx and admin_menu, that effectively makes admin_menu disappear (with default nginx settings).

There is a comment that says to put the setting (admin_menu_cache_client = FALSE) in local.settings.php, but since we want it for all sites I modified the template.

Note that my patch has no code to differentiate nginx (the SERVER_SOFTWARE says HTTP for some reason?), so this could be a very minor performance loss on apache for admin users.

Despite trying, I didn't manage to fix this through nginx vhost settings, but I think the code might just be over my head, so if someone wants to try and do that instead, it might be a better solution.