Adjust to VC changes for ESI marker

The ESI marker has been refactored here:
https://github.com/varnishcache/varnish-cache/commit/8b53dbeda3
This commit reflects this change.

src/vdp_pesi.c

@@ -673,7 +673,7 @@ vdp_pesi_init(VRT_CTX, struct vdp_ctx *vdc, void **priv)
 	}
 
 	RFC2616_Weaken_Etag(vdc->hp);
-	ctx->req->res_mode |= RES_ESI;
+	ctx->req->res_esi = 1;
 	if (*vdc->clen != 0)
 		*vdc->clen = -1;
 
@@ -1294,7 +1294,7 @@ vped_deliver(struct req *req, int wantbody)
 	}
 #endif
 
-	if (gz->is && obj_gzipped && !(req->res_mode & RES_ESI)) {
+	if (gz->is && obj_gzipped && !(req->res_esi)) {
 		/* OA_GZIPBITS are not valid until BOS_FINISHED */
 		if (req->boc != NULL)
 			ObjWaitState(req->objcore, BOS_FINISHED);
@@ -1329,7 +1329,7 @@ vped_deliver(struct req *req, int wantbody)
 		gz = NULL;
 	}
 
-	if (obj_esi && req->res_mode & RES_ESI) {
+	if (obj_esi && req->res_esi) {
 		VSLdbg(req, "vped_deliver: ESI");
 		AZ(vgzgz);