bgpd: add basic support for ETI and ESI for BGP EVPN

bgpd: add basic support for ETI and ESI for BGP EVPN

Ethernet Tag ID (ETI) is part of the prefix. It cannot just be ignored
as it needs to be used when checking for prefix uniqueness. Moreover,
when using Quagga as a route reflector, we need to keep its
value. Therefore, we correctly parse and encode it. We also parse
ESI. While not part of the prefix, it needs to be reflected correctly
by Quagga.

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