Prettyping -6 calls ping6 but newer distributions uses ping -6 and have no ping6 binary

[kuon]

Currently, prettyping -6 calls ping6 binary, but current ping supports a -6 flag directly. On archlinux there is no ping6 binary anymore so prettyping -6 fails.

I think the best fix would be to check if ping6 exists, and if not, call ping -6 instead.