This fixes default packet redirection when merging multiple re6st
networks. Nodes will then install multiple default routes for each re6st
network with the correspond src-prefix.

The changes include:

- redistribute default route with src prefix on gateways
- remove the now obsolete default option
- if subtrees is not available in the kernel, delete src prefix from
  the default route and don't advertise the default route to other
  nodes
- if there is no src prefix in the redistributed default route, add
  one to be retro-compatible with the old gateway behaviour