Merge pull request #6765 from mjstapp/backup_nhg_netlink

lib,zebra: support multiple backup nexthops

diff --cc lib/zclient.c
Simple merge

diff --cc zebra/zapi_msg.c
Simple merge

diff --cc zebra/zebra_dplane.c
Simple merge

diff --cc zebra/zebra_mpls.c
Simple merge

diff --cc zebra/zebra_pw.c
Simple merge

diff --cc zebra/zebra_rib.c
Simple merge

diff --cc zebra/zebra_rnh.c
Simple merge

diff --cc zebra/zebra_vty.c
Simple merge