This allows us to put items on a blacklist for a specific backend based on
the response it gave us. The basic syntax is (vcl_fetch):

if (beresp.status == 500) {
	set beresp.saintmode = 20s;
	restart;
}

Health checks are modified accordingly, which means graced objects are used
if no healthy backends are left to try.

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4208 d4fa192b-c00b-0410-8231-f00ffab90ce4