xwayland: Enable access control on open sockets [CVE-2015-3164 1/3]

Xwayland currently allows wide-open access to the X sockets
it listens on, ignoring Xauth access control.

This commit makes sure to enable access control on the sockets,
so one user can't snoop on another user's X-over-wayland
applications.

Signed-off-by: Ray Strode <rstrode@redhat.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Signed-off-by: Keith Packard <keithp@keithp.com>