Varnish5 builtin VCL without pipe

[Dridi Boukelmoune]

> I'm not sure I get you point about the upgrade field as it's optional.

Like I said, I don't think H2 allows upgrades.

The other question was about backends doing a protocol switch, which
means the client asked for an Upgrade and got it. There's no way to go
from vcl_backend_response to vcl_pipe (or fwiw the state after the
subroutine) so upgrade requests should be intercepted in vcl_recv.

Dridi