[radv] Backport solicited ra unicast to 1.6.x?

[Boris Lytochkin]

Hi.

I am curious if commit 70a4320bdd44122d7a93bc71c77a9d684b3c9adc could be 
cherry-pick'ed into 1.6.x branch?
I've tested it and it's pretty straightforward: the only thing that 
needs to be changed is "current_time()" with "now".
Aside from that, it just works out-of-box.

-- 
Boris Lytochkin
Yandex NOC
+7 (495) 739 70 00 ext 7671