AS690 advisory update

In theory, this would be covered by moving AS279 from the AS macro
for MCI to the AS macro for UUnet.

In current practice, because AS macros are not widely in use (and
aren't in our machine generated aut-num right now), this will have
to be coordinated among providers (like you send us a note saying
your entire AS has moved).

The idea is that this doesn't happen all that often. If it does,
the AS macro is a decent way of handling it.