for sending and instead record the fd+off_t in the storage object.

This eliminates sendfile from storage_file.c, next step is to put
it back in the generic code in cache_response.c





git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@708 d4fa192b-c00b-0410-8231-f00ffab90ce4