Resolve "Neighbor discovery for dhcpv6"

Closes #3217

Remarks:

-The neighbor discovery mechanism requires root privileges (error logged in debug messages).

-Testing of neighbor discovery requires special network with two stations (the network interface does not respond to neighbor soliciation from itself) and works with icmp6, therefore there is no unit test.

-The new configuration parameter neigh-discovery-timeout is added to the interface section. This controls the timeout to wait for neighbor discovery response from clients. The default value 15000 microseconds may be sufficient for ethernet networks, but it is probably too short for wireless networks; see debug messages to optimize the value.