middleware/{log,errors}: output everything to stdout (#684)

Limit the options in both errors and log middleware, just output to
stdout and let someone else (journald,docker) care about where to route
the logs. This removes syslog and logging to a file.

Fixes #573 #602

middleware/log/log.go

@@ -78,8 +78,8 @@ type Rule struct {
 }
 
 const (
-	// DefaultLogFilename is the default log filename.
-	DefaultLogFilename = "query.log"
+	// DefaultLogFilename is the default output name.
+	DefaultLogFilename = "stdout"
 	// CommonLogFormat is the common log format.
 	CommonLogFormat = `{remote} ` + CommonLogEmptyValue + ` [{when}] "{type} {class} {name} {proto} {size} {>do} {>bufsize}" {rcode} {rsize} {duration}`
 	// CommonLogEmptyValue is the common empty log value.