[PATCH] Babel: add RFC9229 (v4 via v6) support

[Juliusz Chroboczek]

> 1) Changed the name of the option to 'extended next hop', for consistency
> with BGP (and in the future also with other protocols). As the option is
> enabled by default, the name likely does not matter that much.

I rather like v4-via-v6, which succintly and clearly states what it is
about.  "Extended next hop" means nothing unless you already know what it
means.

> 3) In babel_handle_update(), move condition 'Reject IPv4 via IPv6 routes
> if disabled' above 'Retraction'. In your patch it was below, so
> retractions in AE 4 were accepted even on interfaces with this feature
> disabled.

That's reasonable enough, it shouldn't matter either way.  Perhaps the
former behaviour was more robust in the case where a node loses its IPv4
address and then immediately retracts the routes that it previously
announced.

Just a minor inconsistency, v4-via-v6 requests are accepted even if the
option is disabled (line 1084).

-- Juliusz