ipv6: disable RFC 4620 nodeinfo by default

RFC 4620 is an experimental RFC that can be used to request information
about a host, including:

• the fully-qualified or single-component name
• some set of the Responder's IPv6 unicast addresses
• some set of the Responder's IPv4 unicast addresses

This is not something that should be made available by default.

PR: 257709
Submitted by: ruben@verweg.com
Reviewed by: melifaro
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39778