bgpd: enhance structures and esi and mac functions to handle EVPN

bgpd: enhance structures and esi and mac functions to handle EVPN

EVPN introduces new objects to be handled: Ethernet Segment Identifier,
and MAC Address.
For Route type 5 message, it also involves gateway IP field that can be
either IPv4 or IPv6 address. This commit introduces those main structures.
It also introduces some functions to convert esi and mac address to
string, and vice-versa.

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