Allow RT::Users->WhoBelongToGroups to optionally return unprivileged users (4.2)

The WhoBelongToGroups filter should really not apply LimitToPrivileged at all, but we can't remove it without breaking security assumptions elsewhere in RT or in 3rd party code. So a new option IncludeUnprivileged is added, a boolean that defaults to false. It can be set to true to disable the LimitToPrivileged filter.

The documentation of the method is amended to explain that it filters out unprivileged members by default.

(See http://issues.bestpractical.com/Ticket/Display.html?id=23089)