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