Added a health parameter for the backend. This is readable in vcl with backend.health.

Added a health parameter for the backend. This is readable in vcl with backend.health.
Made it possible to pass a vcl variable to error (error 200 backend.health).
Implemented a first attempt at an algorithm for checking the health of a backend.
Negative values means the backend has problems, positive values means it is ok. 0 is neutral,
and could mean that it has been a while since the backend was asked for anything.
See the code for details.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@1778 d4fa192b-c00b-0410-8231-f00ffab90ce4