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/man/chasquid.conf.5.pod

@@ -122,6 +122,17 @@ This allows deploying chasquid behind a HAProxy server, as the address
 information is preserved, and SPF checks can be performed properly.
 Default: C<false>.
 
+=item B<smarthost_url> (string):
+
+Smarthost URL. If set, we will send all received email to this location,
+including local mail.
+
+It is of the form C<smtp://user:password@host:port> for SMTP (and STARTTLS
+will be forcefully negotiated), or C<tls://user:password@host:port> for SMTP
+over TLS (usually port 465).
+
+B<EXPERIMENTAL> for now, can change in backwards-incompatible ways.
+
 =back
 
 =head1 SEE ALSO

docs/monitoring.md

@@ -76,6 +76,12 @@ List of exported variables:
   count of STS security checks on outgoing connections, by result (pass/fail).
 - **chasquid/smtpOut/tlsCount** (status -> counter)  
   count of TLS status (insecure TLS/secure TLS/plain) on outgoing connections.
+- **chasquid/smarthostOut/attempts** (counter)  
+  count of attempts to deliver via smarthost.
+- **chasquid/smarthostOut/errors** (reason -> counter)  
+  count of smarthost delivery errors, per reason.
+- **chasquid/smarthostOut/success** (counter)  
+  count of successful delivering via smarthost.
 - **chasquid/sourceDateStr** (string)  
   timestamp when the binary was built, in human readable format.
 - **chasquid/sourceDateTimestamp** (int)  

docs/smarthost.md

@@ -0,0 +1,41 @@
+
+# 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