For XmlRpcGroups (Flotsam) module, when MessageOnlineUsersOnly = true, handle notices to offline users directly as known undeliverable messages rather than discarding or attempting delivery.

Offline notices can still be controlled with the [Messaging] ForwardOfflineGroupMessages setting.
Looks to address more of http://opensimulator.org/mantis/view.php?id=7037
Only for Flotsam now for testing, but if approach works should be possible with core offline notices as well.