mod_disk_cache: NULL fd pointers when closing them, fix missing

close/flush, remove some unneccessary code duplication instead
of calling the right helper in replace_brigade_with_cache().


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@462601 13f79535-47bb-0310-9956-ffa450edef68