xserver/os: safer IPv6 "kame hack" fix, only override sin6_scope_id if zero

This helps on KAME-based systems which want to get rid of this hack.
The assumption is that if sin6_scope_id is set, then the interface index
is no longer embedded in the address.

Signed-off-by: Jeremie Courreges-Anglas <jca@wxcvbn.org>
4 jobs for jca/kame-hack in 10 minutes and 44 seconds
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