spectrumPackages: use link-local router addresses

There's no need for every interface on a router to have it's own
unique IP.  Using link-local addresses, we can assign every interface
on a single router the same IP, and save the effort of
adding/subtracting one all the time to infer the address of one side
of a link from the other.

This means that routers and clients are both only interested in what
the IP of the client should be, since that's the only thing that is
different between different links.  So we change our MAC format to
communicate the client IP to both sides.  If we just made the whole
NIC portion of the MAC the last three octets of an IP address like we
did before, then both ends of the link would have the same MAC, so we
still need to set a bit to encode whether a MAC is at the router side
of the link or the client side.  I've decided to simplify the
implementation by using the most significant bit of the NIC part for
that, and just declaring the rest of that octet to be reserved.  This
way, no calculation is required in the guests -- they just need to
look at the last two octets.  And we can still identify 65536 links in
this scheme, which should be enough for anybody, right?

Message-Id: <20210513201323.2087344-1-hi@alyssa.is>
Reviewed-by: Cole Helbling <cole.e.helbling@outlook.com>

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