The purpose of the this new filter is to override *locally*:
- RTA_SRC ('from' option in ip-route)
- RTA_PREFSRC ('src' in ip-route)

We also force use of ipv6 subtrees because:
- even Linux 2.6.32 has them
- the fallback implementation using a separate table is not equivalent,
  at least not for Nexedi (and we don't need RTA_SRC for ipv4)

TODO: documentation

To be pushed upstream.