Unfucking MP-BGP capability negotiation

protocols/bgp/packet/fuzzing.go

protocols/bgp/server/fsm.go

protocols/bgp/server/fsm_address_family.go

protocols/bgp/server/fsm_established.go

protocols/bgp/server/fsm_open_sent.go

protocols/bgp/server/fsm_open_sent_test.go

protocols/bgp/server/fsm_test.go

protocols/bgp/server/peer.go

protocols/bgp/server/server.go

protocols/bgp/server/update_sender.go

protocols/bgp/server/update_sender_test.go

protocols/bgp/server/withdraw.go

protocols/bgp/server/withdraw_test.go

protocols/bgp/types/options.go