os: support new implicit local user access mode [CVE-2015-3164 2/3]

If the X server is started without a '-auth' argument, then
it gets started wide open to all local users on the system.

This isn't a great default access model, but changing it in
Xorg at this point would break backward compatibility.

Xwayland, on the other hand is new, and much more targeted
in scope.  It could, in theory, be changed to allow the much
more secure default of a "user who started X server can connect
clients to that server."

This commit paves the way for that change, by adding a mechanism
for DDXs to opt-in to that behavior.  They merely need to call

LocalAccessScopeUser()

in their init functions.

A subsequent commit will add that call for Xwayland.

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>