So with clients which only use SLAAC, ctrld could see client's new ip as
soon as its state changes to REACHABLE.
Moreover, the NDP listener is also changed to listen on all possible
ipv6 link local interfaces. That would allow ctrld to get all NDP events
happening in local network.
SLAAC RFC: https://datatracker.ietf.org/doc/html/rfc4862
An interface may have multiple MAC addresses, that leads to the problem
when looking up hostname for its multiple <ip, mac> pairs, because the
"ip" map, which storing "mac => ip" mapping can only store 1 entry. It
ends up returns an empty hostname for a known MAC address.
Fixing this by filling empty hostname based on clients which is already
listed, ensuring all clients with the same MAC address will have the
same hostname information.
The default gateway is usually the DNS server in normal home network
setup for most users. However, there's case that it is not, causing
discover ptr failed.
This commit add discover_ptr_endpoints config parameter, so users can
define what DNS nameservers will be used.
VPN clients often have empty MAC address, because they come from virtual
network interface. However, there's other setup/devices also create
virtual interface, but is not VPN.
Changing source of those clients to empty to prevent confustion in
clients list command output.
So ctrld can record the raw/original client IP instead of looking up
from MAC to IP, which may not the right choice in some network setup
like using wireguard/vpn on Merlin router.
