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

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