lib: add AFI_L2VPN and SAFI_EVPN

lib: add AFI_L2VPN and SAFI_EVPN

To support EVPN, a new AFI and SAFI value are defined here.
For internal processing, two other values are used. Those values will
be used to reach RIB entries by using internal afi and safi values
as indexes. This commit is using naming convention for using EVPN.
External value exchanged in BGP packets  is called of with
IANA_ presence in macro, while internal value will not have _IANA_
presence.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>