enhancement suggestion if -T not given by user

[guo1017138]

As you may know already, if we don't use -T or in previously version (v1.7 and before), this command totally not working because tgt address is "::", such like:
ndptool -t na -U -i eth1 send
The tcpdump result will be:
08:25:06.902409 IP6 (hlim 255, next-header ICMPv6 (58) payload length: 32) toolkit-vm.localdomain > ff02::1: [icmp6 sum ok] ICMP6, neighbor advertisement, length 32, tgt is ::, Flags [override]
destination link-address option (2), length 8 (1): fa:16:3e:03:c1:c2

So, my idea is that, -T should be optional parameter, it -T specified, the behavior should be the same as now. When -T is not offered and also in previously release, ndptool should loop for each IPv6 on this interface and send unsolicited NA one by one.

BR,
Larry