CLEANUP: applet: use the appctx's endp instead of cs->endp

The few applets that set CS_EP_EOI or CS_EP_ERROR used to set it on the
endpoint retrieved from the conn_stream while it's already available on
the appctx itself. Better use the appctx one to limit the unneeded
interactions between the two sides.
diff --git a/src/http_client.c b/src/http_client.c
index 5d22044..bff7057 100644
--- a/src/http_client.c
+++ b/src/http_client.c
@@ -832,8 +832,8 @@
 
 					/* if the request contains the HTX_FL_EOM, we finished the request part. */
 					if (htx->flags & HTX_FL_EOM) {
-						cs->endp->flags |= CS_EP_EOI;
 						req->flags |= CF_EOI;
+						appctx->endp->flags |= CS_EP_EOI;
 						appctx->st0 = HTTPCLIENT_S_RES_STLINE;
 					}