MINOR: spoe: Improve implementation of the payload fragmentation

Now, when a payload is fragmented, the first frame must define the frame type
and the followings must use the special type SPOE_FRM_T_UNSET. This way, it is
easy to know if a fragment is the first one or not. Of course, all frames must
still share the same stream-id and frame-id.

Update SPOA example accordingly.
diff --git a/include/types/spoe.h b/include/types/spoe.h
index a6b0ffc..b65bc7a 100644
--- a/include/types/spoe.h
+++ b/include/types/spoe.h
@@ -294,6 +294,8 @@
 
 /* Frame Types sent by HAProxy and by agents */
 enum spoe_frame_type {
+	SPOE_FRM_T_UNSET = 0,
+
 	/* Frames sent by HAProxy */
 	SPOE_FRM_T_HAPROXY_HELLO = 1,
 	SPOE_FRM_T_HAPROXY_DISCON,