First step of slow client handling: Lose the stevedore function

First step of slow client handling:  Lose the stevedore function
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: svn+ssh://projects.linpro.no/svn/varnish/trunk@708 d4fa192b-c00b-0410-8231-f00ffab90ce4