MINOR: stream-int: rename ->applet to ->appctx

Since this is the applet context, call it ->appctx to avoid the confusion
with the pointer to the applet. Many places were changed but it's only a
renaming.
diff --git a/include/types/stream_interface.h b/include/types/stream_interface.h
index 1af5be5..b7f1a5a 100644
--- a/include/types/stream_interface.h
+++ b/include/types/stream_interface.h
@@ -169,7 +169,7 @@
 	/* struct members below are the "remote" part, as seen from the buffer side */
 	int conn_retries;	/* number of connect retries left */
 	int send_proxy_ofs;	/* <0 = offset to (re)send from the end, >0 = send all */
-	struct appctx applet;   /* context of the running applet if any */
+	struct appctx appctx;   /* context of the running applet if any */
 };
 
 /* An applet designed to run in a stream interface */