courier: Use the hostname in SMTP HELO

The SMTP courier, which handles outgoing connections, uses the domain of
the envelope's from as the domain in the HELO/EHLO greeting.

This works fine in practice, but ideally the domain used in the greeting
should match the reverse DNS record. This used to be more relevant but
nowadays it is not really enforced; however, it sometimes comes up in
self checks, and might cause some confusion when troubleshooting.

So this patch makes it use the configured hostname instead, which is
under the users' control and more likely to be compliant. It also
simplifies the code.

The documentation of the hostname configuration option is also updated
to mention this behaviour.

Thanks to Jonas Seydel (thor77) for bringing this up.

internal/config/config.proto

@@ -3,7 +3,11 @@ syntax = "proto3";
 
 message Config {
 	// Default hostname to use when saying hello.
-	// This is used to say hello to clients, for aesthetic purposes.
+	// This is used:
+	// 1) To say hello to clients, for aesthetic purposes.
+	// 2) As the HELO/EHLO domain on outgoing SMTP connections, so ideally
+	//    it would resolve back to the server. In practice, it's not a big
+	//    deal if it isn't, but it makes troubleshooting easier.
 	// Default: the system's hostname.
 	string hostname = 1;