Don my peril sensitive sun-glasses, and go over the TTL calculation

Don my peril sensitive sun-glasses, and go over the TTL calculation
routine.

Follow RFC2616 more closely, and make relative (ie: max-age) specifications
take precedence over absolute specifications (Expires:)

Add an explicit clock-skew window (parameter: clock_skew) and only
interpret Expires: in absolute terms if the Date: header is inside
this window.

Explicit check for the case where the backend sends an Expires: which
is before the Date:, even if the Date: is out side our window.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@3019 d4fa192b-c00b-0410-8231-f00ffab90ce4