WIP: Add smarthost support

WORK IN PROGRESS -- WORK IN PROGRESS -- WORK IN PROGRESS

This patch adds support for delivering mail via a smarthost.

In this mode, all accepted mail gets delivered through an SMTP
connection to a specific host, statically configured.

docs/smarthost.md

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+
+# Smarthost client mode
+
+As of version 1.6 (2020-XX), [chasquid] supports operating as a [smarthost]
+client.
+
+In this mode, chasquid will deliver all accepted mail (both local and remote)
+to a single specific host (the *smarthost* server).
+
+## Status
+
+It is **EXPERIMENTAL** for now. The configuration options and behaviour can
+change in backwards-incompatible ways.
+
+
+## Security
+
+chasquid will always negotiate TLS on the connection to the smarthost, and
+expects a valid certificate.
+
+If TLS is not available, or the certificate is not valid, the mail will remain
+in the queue and will not be delivered.
+
+
+## Configuring
+
+Add the following line to `/etc/chasquid/chasquid.conf`:
+
+```
+smarthost_url: "smtp://user:password@server:587"
+```
+
+Replace `user` and `password` with the credentials used to authenticate to the
+smarthost server, and `server:587` with the server address, including port.
+
+You can also use the `tls` scheme for direct TLS connections (usually on port
+465).
+
+
+[chasquid]: https://blitiri.com.ar/p/chasquid
+[smarthost]: https://en.wikipedia.org/wiki/Smart_host