When sending group notices through group messaging, allow the agent ID to use for fetching group data to be different from im.fromAgentID

This is because xmlrpcgroups currently always checks visibility for the requesting agent ID (unlike Groups v2, which can accept UUID.Zero)
But group notice IMs have a from agent which is the group rather than the sending agent.
Further addresses http://opensimulator.org/mantis/view.php?id=7037