CLEANUP: stconn: rename cs_cant_get() to se_need_more_data()

An equivalent applet_need_more_data() was added as well since that function
is mostly used from applet code. It makes it much clearer that the applet
is waiting for data from the stream layer.
diff --git a/src/http_client.c b/src/http_client.c
index da774e7..ee0c69a 100644
--- a/src/http_client.c
+++ b/src/http_client.c
@@ -1018,7 +1018,7 @@
 	s->res.flags |= CF_READ_DONTWAIT;
 
 	/* applet is waiting for data */
-	cs_cant_get(s->scf);
+	applet_need_more_data(appctx);
 	appctx_wakeup(appctx);
 
 	hc->appctx = appctx;