bgpd: add an option for RT auto-derivation to use RFC 8635.

bgpd: add an option for RT auto-derivation to use RFC 8635.

RFC 8635 explains how RT auto-derivation should be done in section
5.1.2.1 [1]. In addition to encoding the VNI in the lowest bytes, a
3-bit field is used to encode a namespace. For VXLAN, we have to put 1
in this field. This is needed for proper interoperability with RT
auto-derivation in JunOS. Since this would break existing setup, an
additional option, "autort rfc8365-compatible" is used.

[1]: https://tools.ietf.org/html/rfc8365#section-5.1.2.1

Signed-off-by: Vincent Bernat <vincent@bernat.im>