ntpd: Support IPv6-only hosts

0.pool.* returns only IPv4 addresses.
2.pool.* returns both, IPv6 and IPv4 addresses.

conservatively extend our IPv4 only pool configuration by adding a
second pool, which also returns IPv6 addresses.

PR: 270536
Reported by: Lapo Luchini <lapo@lapo.it>
MFC after: 3 days
Pull Request: https://github.com/freebsd/freebsd-src/pull/731
Differential Revision: https://reviews.freebsd.org/D39954