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/C/chasquid.conf

@@ -0,0 +1,10 @@
+smtp_address: ":3025"
+submission_address: ":3587"
+submission_over_tls_address: ":3465"
+monitoring_address: ":3099"
+
+mail_delivery_agent_bin: "test-mda"
+mail_delivery_agent_args: "%to%"
+
+data_dir: "../.data-C"
+mail_log_path: "../.logs/mail_log-C"