Add fix for VuXML ID: ff00f2ce-c54c-11d8-b708-00061bc2ad93
When the IPv6 code was added to xdm a critical test to disable xdmcp was
accidentally removed. This caused xdm to create the chooser socket regardless
if DisplayManager.requestPort was disabled in xdm-config or not.
Prodded by: Dejan Lesjak
Obtained from: XFree86 bugzilla / X.Org CVS