ip/dhcp6d: work arround non-ethernet based client duid

ip/dhcp6d: work arround non-ethernet based client duid

in ndb, we use the ethernet mac to identify the client.
in dhcpv6, there is just a uniqueue device id that
might even be generated randomly. to find the ethernet
address of a client, check the duid type and only use
it when the dudid is of type 1 (link layer) or 3 (link
layer address + time) and the link layer address type
is 1 (ethernet). otherwise, assume the source ip is
a link local address and extract it from that.

this hack works for thinkpad t495, which uses random
uuid based client duid.