ping: Consistently use EX_NOHOST for DNS failures.

Traditionally, ping returned exit code EX_NOHOST if a DNS lookup failed.
That is still the case for the legacy code in the new merged ping, but
not for IPv6 targets, nor when a DNS lookup is performed in order to
determine which version of the tool to invoke.

While here, also make sure that the error message is consistent.

Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D42159