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: http://www.varnish-cache.org/svn/trunk/varnish-cache@1778 d4fa192b-c00b-0410-8231-f00ffab90ce4