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/A/chasquid.conf

@@ -0,0 +1,17 @@
+smtp_address: ":1025"
+submission_address: ":1587"
+submission_over_tls_address: ":1465"
+monitoring_address: ":1099"
+
+# We don't expect this to be used, pick something that will error to ease
+# troubleshooting.
+mail_delivery_agent_bin: "no_mda_needed"
+
+data_dir: "../.data-A"
+mail_log_path: "../.logs/mail_log-A"
+
+# srv-b is our smarthost.
+# We use tls protocol because the smtp one is already well exercised in the
+# package tests.
+smarthost_url: "tls://userB@srv-b:userB@srv-b:2465"
+