Enforce a minimum ttl for "hit for pass" objects to prevent a value

Enforce a minimum ttl for "hit for pass" objects to prevent a value
of zero from serializing access to an object with very draconian
backend cache-control headers.

We could get far even with a one second TTL, but following our general
"there is a reason people put Varnish there in the first place" logic
we use the default_ttl parameter (default: 120 s) for this value.

If another value is desired, this can be set in vcl_fetch, even if it
looks somewhat counter-intuitive:

	sub vcl_fetch {
		if (obj.http.set-cookie) {
			set obj.ttl = 10s;
			pass;
		}
	}

Fixes #425

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@3537 d4fa192b-c00b-0410-8231-f00ffab90ce4