logrotate: Never delete system log messages /var/log/messages

Currently, they are deleted after twelve months, which is undesired, as
older logs are sometimes needed for comparison and debugging.

> If count is -1, old logs are not removed at all (use with caution, may
> waste performance and disk space).

We have enough space, so keep the logs.

logrotate/logrotate.conf

@@ -2,7 +2,7 @@
 	dateext
 	dateformat .%Y-%m
 	dateyesterday
-	rotate 12
+	rotate -1
 	monthly
 	sharedscripts
 	postrotate