bgpd(8) will soon support ADD-PATH (RFC7911) and enhanced route refresh

bgpd(8) will soon support ADD-PATH (RFC7911) and enhanced route refresh
(RFC7313). This is the frist step toward this.

It adds the capability parsers for the two no capabilities, extends the
capability struct and adds the capability negotiation bits.
The route refresh message parser and generator are extended to support
the BoRR and EoRR message. Also add the new NOTIFICATION type and subtype
for the route refresh message.