[ipv6] Allow for multiple routers

Select the IPv6 source address and corresponding router (if any) using
a very simplified version of the algorithm from RFC6724:

- Ignore any source address that has a smaller scope than the
  destination address.  For example, do not use a link-local source
  address when sending to a global destination address.

- If we have a source address which is on the same link as the
  destination address, then use that source address.

- If we are left with multiple possible source addresses, then choose
  the address with the smallest scope.  For example, if we are sending
  to a site-local destination address and we have both a global source
  address and a site-local source address, then use the site-local
  source address.

- If we are still left with multiple possible source addresses, then
  choose the address with the longest matching prefix.

For the purposes of this algorithm, we treat RFC4193 Unique Local
Addresses as having organisation-local scope.  Since we use only
link-local scope for our multicast transmissions, this approximation
should remain valid in all practical situations.

Originally-implemented-by: Thomas Bächler <thomas@archlinux.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

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