ping: add a ping6 hard link for backwards compatibility

[asomers]

Summary

ping: add a ping6 hard link for backwards compatibility

When invoked as "ping6", ping will now attempt to use ICMPv6 for hostnames
that resolve both IPv4 and IPv6 addresses.

Test Plan

Test case added

[manu]

LGTM
Thanks

[bz]

Ignoring the test case (but thanks for adding it) this looks good to me.
Thank you for bringing it back so quickly.

You may or may not merge the two #ifdef INET6 blocks; either way, go ahead please and commit it.

sbin/ping/main.c

76

I wonder if you can merge the two INET6 blocks as

if (strcmp(getprogname(), "ping6") == 0)
   ipv6 = true;
else
   ipv6 = false;
#endif
   ipv4 = false;

[asomers]

Fixed by r368078