- Rewrote whole interface logic. Removed support for multiple addresses per

- Rewrote whole interface logic. Removed support for multiple addresses per
  interface since it makes much trouble everywhere. Instead, we understand
  secondary addresses as subinterfaces.

- In case interface addresses or basic flags change, we simply convert it
  to a down/up sequence.

- Implemented the universal neighbour cache. (Just forget what did previous
  includes say of neighbour caching, this one is brand new.)