[MINOR] add the "force-persist" statement to force persistence on down servers

This is used to force access to down servers for some requests. This
is useful when validating that a change on a server correctly works
before enabling the server again.
diff --git a/include/types/proxy.h b/include/types/proxy.h
index a4c99cb..c4fb505 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -171,6 +171,7 @@
 	struct list block_cond;                 /* early blocking conditions (chained) */
 	struct list redirect_rules;             /* content redirecting rules (chained) */
 	struct list switching_rules;            /* content switching rules (chained) */
+	struct list force_persist_rules;        /* 'force-persist' rules (chained) */
 	struct list sticking_rules;             /* content sticking rules (chained) */
 	struct list storersp_rules;             /* content store response rules (chained) */
 	struct {                                /* TCP request processing */
@@ -297,6 +298,11 @@
 	} be;
 };
 
+struct force_persist_rule {
+	struct list list;			/* list linked to from the proxy */
+	struct acl_cond *cond;			/* acl condition to meet */
+};
+
 struct sticking_rule {
 	struct list list;                       /* list linked to from the proxy */
 	struct acl_cond *cond;                  /* acl condition to meet */