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.

test/t-20-smarthost/msmtprc

@@ -0,0 +1,14 @@
+account default
+
+host srv-A
+port 1587
+
+tls on
+tls_trust_file A/certs/srv-A/fullchain.pem
+
+from userA@srv-A
+
+auth on
+user userA@srv-A
+password userA
+