ipneigh: add support to print brief output of neigh cache in tabular format

ipneigh: add support to print brief output of neigh cache in tabular format

Make use of the already available brief flag and print the basic details of
the IPv4 or IPv6 neighbour cache in a tabular format for better readability
when the brief output is expected.

$ ip -br neigh
172.16.12.100                           bridge0          b0:fc:36:2f:07:43
172.16.12.174                           bridge0          8c:16:45:2f:bc:1c
172.16.12.250                           bridge0          04:d9:f5:c1:0c:74
fe80::267b:9f70:745e:d54d               bridge0          b0:fc:36:2f:07:43
fd16:a115:6a62:0:8744:efa1:9933:2c4c    bridge0          8c:16:45:2f:bc:1c
fe80::6d9:f5ff:fec1:c74                 bridge0          04:d9:f5:c1:0c:74

And add "ip neigh show" to the list of ip sub commands mentioned in the man
page that support the brief output in tabular format.

Signed-off-by: Gokul Sivakumar <gokulkumar792@gmail.com>
Signed-off-by: David Ahern <dsahern@kernel.org>