By reusing recently added "split TAP" infrastructure we can adjust dnsmasq
configuration to provide unique IPv6 to each RU.

- ru_mac_addr becomes per-RU setting and without default. We talked with Lu, and
  since now all Lopcomm units are shipped from the factory with unique MAC, it
  both does not make sense to provide the default, and we can rely on all units
  to have different MACs and configure DHCP replies based on that.

- No need to provide /64 network to every RU. We cannot actually do that anyway
  because normally SlapOS provides /71 address range for its slaptap. In the
  new configuration everything works with smaller networks just ok.

/cc @jhuge, @xavier_thompson, @Daetalus
/reviewed-by @lu.xu
/reviewed-on nexedi/slapos!1472