replace ping6 with ping

reason: after the upgrade of iputils from 20151218 to 20161105
functionality of ping6 and tracepath6 was merged into ping and tracepath.

Ping is now mostly a drop-in replacment for ping6, except that selecting a
specific interface is done by encoding it into the address (ex.: fe80::1%eth0)
rather then specifing it with the `-I` flag.