[ping] Fix privileged IPv4 ping (RAW socket).
[ping] Fix privileged IPv4 ping (RAW socket).

= Privileged IPv4 ping was broken on Linux (#122). For IPv4 RAW sockets, IP header is not stripped.
= If probe is IPv4, and incoming packet's first byte suggests that there is an IP header, assume that regardless of the OS. I think it's a safe assumption.

cloudprober/cloudprober,cloudprober/cloudprober,cloudprober/cloudprober