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.

internal/config/config.proto

@@ -59,5 +59,10 @@ message Config {
 	// "u.se_r" will be delivered to "user".
 	// Default: ".".
 	string drop_characters = 10;
+
+	// Path where to write the mail log to.
+	// If "<syslog>", log using the syslog (at MAIL|INFO priority).
+	// Default: <syslog>
+	string mail_log_path = 11;
 }