MEDIUM: stats: don't use conn->xprt_st anymore

We're trying to move the applets out of the struct connection. So
let's remove the dependence on xprt_st and introduce si->applet.st2
to store the missing contextual data instead.
diff --git a/include/types/stream_interface.h b/include/types/stream_interface.h
index fe49371..5ce65a6 100644
--- a/include/types/stream_interface.h
+++ b/include/types/stream_interface.h
@@ -117,7 +117,10 @@
 	int conn_retries;	/* number of connect retries left */
 	int send_proxy_ofs;	/* <0 = offset to (re)send from the end, >0 = send all */
 	struct {
-		unsigned int st0, st1;     /* may be used by any function above */
+		unsigned int st0;     /* CLI state for stats, session state for peers */
+		unsigned int st1;     /* prompt for stats, session error for peers */
+		unsigned int st2;     /* output state for stats, unused by peers  */
+
 		union {
 			struct {
 				struct proxy *px;