Allow variables for "mogilefs_domain", also make the directive mandatory.

Example configuration:

location ~ ^/(?<domain>[^/]+)/(?<class>[^/]+)/(?<key>.+)$ {
    mogilefs_tracker 127.0.0.1:7001;
    mogilefs_domain $domain;
    mogilefs_methods GET PUT DELETE;
    mogilefs_class $class;

    mogilefs_pass $key {
        proxy_pass $mogilefs_path;
        proxy_hide_header Content-Type;
        proxy_buffering off;
    }
}
Fix -Werror=unused-but-set-variable warnings with newer gcc

They're especially nasty with -Werror.