ip6addrctl: Update the sample configuration file

Update the sample ip6addrctl.conf.sample file to match the default
policy, currently based on RFC 6724.

MFC after: 3 days
Signed-off-by: Jose Luis Duran <jlduran@gmail.com>

Reviewed by: imp, glebius
Pull Request: https://github.com/freebsd/freebsd-src/pull/1375

(cherry picked from commit 6bfbfc8f4fdc6ccd200541ef738a90d1be3c1ab7)