maillog: Introduce a special-purpose mail logging package

The default INFO logs are more oriented towards debugging and can be
a bit too verbose when looking for high-level information.

This patch introduces a new "maillog" package, used to log messages of
particular relevance to mail transmission at a higher level.

test/t-03-queue_persistency/config/chasquid.conf

@@ -6,3 +6,4 @@ mail_delivery_agent_bin: "test-mda"
 mail_delivery_agent_args: "%to%"
 
 data_dir: "../.data"
+mail_log_path: "../.logs/mail_log"