Many clients don't include SLLA option in router solicitations:
https://github.com/radvd-project/radvd/issues/63#issuecomment-287172252
Answer these router solicitations by triggering a neighbour
solicitation when SLLA is not included, and then waiting for the
neighbour advertisment.

We were using the global value, not the one passed in parameter.

Thanks to Antonin Décimo.

Thanks to Antonin Décimo.

Sometimes the desired ancillary message does not come head-first.

If there are 16 slots, then the maximum index is 15.

This simplifies the code a lot, since it gives us the notion of a session
for free.  And congestion control, of course.

This is not quite ready yet: we don't wait for the peer's handshake before
initiating flooding, and we don't perform duplicate suppression.  Doing that
will require a different data structure.