* httpcaddyfile: Add shortcut for proxy hostport placeholder
I've noticed that it's a pretty common pattern to write a proxy like this, when needing to proxy over HTTPS:
```
reverse_proxy https://example.com {
header_up Host {http.reverse_proxy.upstream.hostport}
}
```
I find it pretty hard to remember the exact placeholder to use for this, and I continually need to refer to the docs when I need it. I think a simple fix for this is to add another Caddyfile placeholder for this one to shorten it:
```
reverse_proxy https://example.com {
header_up Host {proxy_hostport}
}
```
* Switch the shortcut name
ab32440b21