traceroute6(8): Stop probing on networks rejected.

Stop probing after receiving ICMP6_DST_UNREACH and print
ICMP6 error code.

The behavior can be observed by tracing the route to,
for example, 2a02:ee80:4028:1126::

Discussed with: Job Snijders, Nick Hilliard
Obtained from: OpenBSD
Reviewed by: markj, jlduran
Differential Revision: https://reviews.freebsd.org/D47520