MINOR: applet: Add appctx_init() helper fnuction

It is just a helper function that call the .init applet callback function,
if it exists. This will simplify a bit the init stage when a new applet is
started. For now, this callback function is only used when a new service is
started.
diff --git a/src/stream.c b/src/stream.c
index dd3358e..29d1a60 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -997,9 +997,8 @@
 			return ACT_RET_ERR;
 
 		/* Finish initialisation of the context. */
-		memset(&appctx->ctx, 0, sizeof(appctx->ctx));
 		appctx->rule = rule;
-		if (appctx->applet->init && !appctx->applet->init(appctx))
+		if (appctx_init(appctx) == -1)
 			return ACT_RET_ERR;
 	}
 	else