Merge pull request #5737 from mjstapp/zebra_disable_kern_nhs

zebra: add config to disable use of kernel nexthops

diff --cc zebra/rt_netlink.c
Simple merge

diff --cc zebra/zebra_nhg.c
Simple merge

diff --cc zebra/zebra_nhg.h
Simple merge

diff --cc zebra/zebra_vty.c
Simple merge