ping: do reverse DNS lookup of the target address

When printing replies, ping will now attempt a reverse DNS lookup of the
target. That can be suppressed by using the "-n" option. Curiously, ping
has always done reverse lookups in certain error paths, but never in the
success path.

Submitted by: Ján Sučan <sucanjan@gmail.com>
MFC after: 2 weeks
Sponsored by: Google LLC (Google Summer of Code 2019)
Differential Revision: https://reviews.freebsd.org/D21351