[LightDM] Error connecting to X server via IPv6

[Patrick Monnerat]

Hi list,

I've discovered a problem in lightdm XDMCP using link-local IPv6 
addresses without interface index to connect to X server.
It has been originally reported at 
https://bugzilla.redhat.com/show_bug.cgi?id=1322775.

The problem can be fixed by having the XDMCP client not transmitting 
link-local addresses in the request, but if it does, lightdm XDMCP 
server can be a lot smarter if the attached patch is applied.

It changes the X server address selection order to set the lowest 
preference to link-local addresses, even if matching the source address 
or family.

This is not 100% full-proof: if only link-local addresses have been 
received, one of them is used anyway. In this case, a complete solution 
would be to determine and set the address interface index if not ambiguous.

However, this patch fixes the problem for most of the non-pathological 
cases.

Regards,
Patrick
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