MINOR: applet: add init function

this patch adds init function for the applet. the HAProxy standard
applet (peers, stats, lua) have no standard way for the applet
initialisation. This new function pointer have for goal to try to
standardize the applet initialization.
diff --git a/include/types/applet.h b/include/types/applet.h
index 6b46f49..13817b2 100644
--- a/include/types/applet.h
+++ b/include/types/applet.h
@@ -24,6 +24,8 @@
 
 #include <types/hlua.h>
 #include <types/obj_type.h>
+#include <types/proxy.h>
+#include <types/stream.h>
 #include <common/chunk.h>
 #include <common/config.h>
 
@@ -34,6 +36,8 @@
 	enum obj_type obj_type;            /* object type = OBJ_TYPE_APPLET */
 	/* 3 unused bytes here */
 	char *name;                        /* applet's name to report in logs */
+	int (*init)(struct appctx *, struct proxy *px, struct stream *strm);   /* callback to init ressources, may be NULL.
+	                                     expect 1 if ok, 0 if an error occurs, -1 if miss data. */
 	void (*fct)(struct appctx *);      /* internal I/O handler, may never be NULL */
 	void (*release)(struct appctx *);  /* callback to release resources, may be NULL */
 };