Update macro list

Put in correct number of possible `$USERx$` variables.

For what it's worth here is the same issue on the Nagios tracker which I don't think has been fixed yet:

http://tracker.nagios.org/view.php?id=557

Here is a thread about the issue as well:

http://support.nagios.com/forum/viewtopic.php?f=7&t=24946

Also, confirmed the allowed `$USERx$` macros is actually `256` in Naemon:

https://github.com/naemon/naemon-core/blob/master/naemon/macros.h#L14