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

[Juliusz Chroboczek]

> I just though that the default value for the option is enabled, but
> perhaps it should be enabled only if such routes are supported by
> platform code (i.e. enabled on Linux, but disabled on BSD, as we do
> not support such routes on BSD).

IMHO, it should not be possible to enable v4-via-v6 unless the platform
supports it.  Otherwise, you risk creating persistent blackholes.

(There's also the PMTUD problem described in RFC 9229 Section 3.)

-- Juliusz