Add default log_format to the config

conf/logging.inc

@@ -1,3 +1,9 @@
 # Included from top-level .conf file
 
 level = "info";
+log_format =<< EOD
+id: <$mid>,$if_qid{ qid: <$>,}$if_ip{ ip: $,}$if_user{ user: $,}$if_smtp_from{ from: <$>,}
+(default: $is_spam ($action): [$scores] [$symbols]),
+len: $len, time: $time_real real,
+$time_virtual virtual, dns req: $dns_req
+EOD