Apply the 3 least significant digit rule of status code on H/2

In VCL we allow status codes larger then 999, and do modulo 1000 when
sending it to the client. But the code to apply the modulo was missing in
H/2.

This issue was uncovered and reported by @Francois-v3.

Fixes first case of: #3301

Conflicts:
    bin/varnishd/http2/cache_http2_deliver.c

bin/varnishd/http2/cache_http2_deliver.c

@@ -238,7 +238,7 @@ h2_build_headers(struct vsb *resp, struct req *req)
 
 	AN(VSB_new(resp, req->ws->f, l, VSB_FIXEDLEN));
 
-	l = h2_status(buf, req->resp->status);
+	l = h2_status(buf, req->resp->status % 1000);
 	VSB_bcat(resp, buf, l);
 
 	hp = req->resp;