netinet6: fix ndp proxying

We could insert proxy NDP entries by the ndp command, but the host
with proxy ndp entries had not responded to Neighbor Solicitations.
Change the following points for proxy NDP to work as expected:

• join solicited-node multicast addresses for proxy NDP entries in order to receive Neighbor Solicitations.
• look up proxy NDP entries not on the routing table but on the link-level address table when receiving Neighbor Solicitations.

Reviewed By: melifaro
Differential Revision: https://reviews.freebsd.org/D35307
MFC after: 2 weeks