reverseproxy: Allow using TLS for port 80 upstreams (see #3361)
reverseproxy: Allow using TLS for port 80 upstreams (see #3361)

An upstream like https://localhost:80 is still forbidden, but an addr of
localhost:80 can be used while explicitly enabling TLS as an override;
we just don't allow the implicit behavior to be ambiguous.

abiosoft/caddy,abiosoft/caddy,mholt/caddy,mholt/caddy,l3dlp/caddy,l3dlp/caddy,l3dlp/caddy