hostapd: Support showing neighbor list through hostapd_cli

hostapd: Support showing neighbor list through hostapd_cli

This lets one know the current neighbor list, and could be used
to populate the neighbor list of other hostapd processes.

For instance:

$ hostapd_cli -i vap0001 show_neighbor
04:f0:21:1e:ae:b0 ssid=04f0211eaeb0af190000802809 nr=04f0211eaeb0af1900008028090603022a00
$ hostapd_cli -i vap0000 set_neighbor 04:f0:21:1e:ae:b0 ssid=04f0211eaeb0af190000802809 nr=04f0211eaeb0af1900008028090603022a00
OK
$ hostapd_cli -i vap0000 show_neighbor
04:f0:21:1e:ae:b0 ssid=04f0211eaeb0af190000802809 nr=04f0211eaeb0af1900008028090603022a00
04:f0:21:c3:b2:b0 ssid=04f021c3b2b0af190000802809 nr=04f021c3b2b0af1900008028090603022a00

Signed-off-by: Ben Greear <greearb@candelatech.com>