MEDIUM: session: implement the "use-server" directive

Sometimes it is desirable to forward a particular request to a specific
server without having to declare a dedicated backend for this server. This
can be achieved using the "use-server" rules. These rules are evaluated after
the "redirect" rules and before evaluating cookies, and they have precedence
on them. There may be as many "use-server" rules as desired. All of these
rules are evaluated in their declaration order, and the first one which
matches will assign the server.
diff --git a/src/proxy.c b/src/proxy.c
index 84d430a..7f15bb2 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -427,6 +427,7 @@
 	LIST_INIT(&p->redirect_rules);
 	LIST_INIT(&p->mon_fail_cond);
 	LIST_INIT(&p->switching_rules);
+	LIST_INIT(&p->server_rules);
 	LIST_INIT(&p->persist_rules);
 	LIST_INIT(&p->sticking_rules);
 	LIST_INIT(&p->storersp_rules);