a body in the beresp up before the vcl_fetch{} call and prepare to
make it possible to modify the value from VCL.

Move the actual code to rfc2616.c along with the default TTL determination.



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5477 d4fa192b-c00b-0410-8231-f00ffab90ce4