Stop currently unsettable display names from appearing when [ClientStack.LindenCaps] Cap_GetDisplayNames = "localhost" is set by never passing the username.

Unexpectedly, not setting user name achieves this whereas one might have expected it to be done by not setting display_name in the cap OSD.
Emulates LL behaviour when no display name is set.
Relates to http://opensimulator.org/mantis/view.php?id=7408

OpenSim/Capabilities/Handlers/GetDisplayNames/GetDisplayNamesHandler.cs

@@ -97,7 +97,7 @@ namespace OpenSim.Capabilities.Handlers
                         osdname["display_name"] = OSD.FromString(name);
                         osdname["legacy_first_name"] = parts[0];
                         osdname["legacy_last_name"] = parts[1];
-                        osdname["username"] = OSD.FromString(name);
+                        osdname["username"] = "";
                         osdname["id"] = OSD.FromUUID(uuid);
                         osdname["is_display_name_default"] = OSD.FromBoolean(false);