unknown config block: local_tls

# Describe the bug

Cannot run maddy with tls.loader.acme in config, getting "unknown config block: local_tls" error on start.

# Steps to reproduce

- get latest code from git.
- in maddy.conf replace `tls file /etc/maddy/certs/$(hostname)/fullchain.pem /etc/maddy/certs/$(hostname)/privkey.pem` with

```
tls.loader.acme local_tls {
email put-your-email-here@example.org
agreed # indicate your agreement with Let's Encrypt ToS
challenge dns-01
}

tls &local_tls
```
_this is taken from `https://maddy.email/reference/tls-acme/`_

- start maddy

# Log files

```unknown config block: local_tls```

# Configuration file

see above in steps

# Environment information

* maddy version: master

Start by reproducing the configuration in maddy.conf using the tls.loader.acme local_tls block and the tls &local_tls reference. Trace how these configuration entry points are parsed and compare them with the tls-acme reference; done means maddy starts without the unknown config block error.

Scritto dal modello di indicizzazione a partire dal testo della issue.