MINOR: lua: add AppletTCP class and service

This class is used by Lua code for running as an applet called in TCP mode.
It defines also the lua service.
diff --git a/include/types/hlua.h b/include/types/hlua.h
index 1bf2a75..eccfc6c 100644
--- a/include/types/hlua.h
+++ b/include/types/hlua.h
@@ -17,6 +17,7 @@
 #define CLASS_CHANNEL      "Channel"
 #define CLASS_HTTP         "HTTP"
 #define CLASS_MAP          "Map"
+#define CLASS_APPLET_TCP   "AppletTCP"
 
 struct stream;
 
@@ -98,6 +99,13 @@
 	struct proxy *p;
 };
 
+/* This struct contains the applet context. */
+struct hlua_appctx {
+	struct appctx *appctx;
+	luaL_Buffer b; /* buffer used to prepare strings. */
+	struct hlua_txn htxn;
+};
+
 /* This struc is used with sample fetches and sample converters. */
 struct hlua_smp {
 	struct stream *s;