zebra: ZEBRA_HELLO and mopping up routes (BZ#448)

ZEBRA_HELLO message is used by routing daemons to inform zebra
what type of routes daemon will be announcing to zebra. Also
zebra uses route_type_oaths array to track which daemon announces
which protocol. Zebra mops up routes if daemon didn't for some
reason.
diff --git a/lib/zebra.h b/lib/zebra.h
index 4b4c7c0..ab072d6 100644
--- a/lib/zebra.h
+++ b/lib/zebra.h
@@ -417,7 +417,8 @@
 #define ZEBRA_ROUTER_ID_ADD               20
 #define ZEBRA_ROUTER_ID_DELETE            21
 #define ZEBRA_ROUTER_ID_UPDATE            22
-#define ZEBRA_MESSAGE_MAX                 23
+#define ZEBRA_HELLO                       23
+#define ZEBRA_MESSAGE_MAX                 24
 
 /* Marker value used in new Zserv, in the byte location corresponding
  * the command value in the old zserv header. To allow old and new