This allows using telnet on the local interface.

It's not like it's run that often, but I've got OCD.

This is sometimes useful after a mobility event, but it increases the
amount of control traffic when a route is unstable.  Let the periodic
update take care of mobility.

This never triggerred, since send_ihu is never called with no interface.

The source table is now sorted, just like the route table.  Source insertion
is still linear, and source expiry quadratic, but this hopefully doesn't
matter much.

This appears to get rid with ENOBUFS on recvmsg.  Kill me if I know why.

Two main modifications:
  - source-specific routes are putted in front of the RIB,
  - route_stream now allows traversing all routes, all installed routes,
    or source-specific installed routes.