caddyfile: Stricter parsing, error for brace on new line

[francislavoie]

Feedback from https://caddy.community/t/caddy-as-an-authentication-proxy-to-secure-endpoint/19691 made me realize we should be a bit stricter with parsing braces.

This makes this config cause an error:

:8001 {
    reverse_proxy https://example.com 
    {
        header_up Host {upstream_hostport}
    }
}

Error: Caddyfile.test3:3 - Error during parsing: Unexpected '{' on a new line; did you mean to place the '{' on the previous line?

The other changes to reverse_proxy/php_fastcgi/forward_auth were to fix the error expecting 'https://' but got '://' from that thread; essentially, the upstreams were being parsed for each iteration of the unmarshal loop with RemainingArgs() but that doesn't make sense. Changing the loop means both the shortcut directives would misbehave due to the dispenser cursor being in the wrong place, so they need to be advanced before calling down.

[mholt]

This is great. Wow. Thanks for the improvement!!