nd6_rtr:

Pull in the TAILQ_HEAD() as it is not needed outside nd6_rtr.c.
Rename the TAILQ_HEAD() struct and the nd_defrouter variable from
"nd_" to "nd6_" as they are not part of the RFC 3542 API which uses "ND_".

Ideally I'd like to also rename the struct nd_defrouter {} to "nd6_*"
but given that is used externally there is more work to do.

No functional changes.

MFC after: 3 weeks
Sponsored by: Netflix