Re6st protocol 8: redistribute default route with src prefix

See: https://www.erp5.com/project_section/vifib/forum/Re6st-and-BGP-cqG5PYlfwF

re6st protocol 8: redistribute default route with src prefix

- redistribute default route with src prefix
- remove default option
- if subtree is not available, delete src prefix from default route
- if no src prefix in redistributed default route, add one to be
  retro-compatible

I tested this with demo, with unittests, and with a package compiled on OBS.

Before merging, https://lab.nexedi.com/nexedi/babeld/tree/bgp needs to be merged and a new tag should be created (should the tag be named hmac-nxd4 ?)

/cc @jm @tomo