Commit d046b245 introduces a bug which causes ping to always fail.
The source of this bug is:
```
// Check if reply from same ID
body := m.Body.(*icmp.Echo)
if body.ID != p.id {
return nil
}
```
Which due to the selection of p.id requires that SetPrivileged is
set to true. In the case where Privileged (i.e p.network == udp)
it is left to the kernel to set the ICMP id.
https://lwn.net/Articles/443051/ Discusses the introduction of
non-setuid-less ping. The kernel implementation for this
interface dictates using the local port, which gets mapped into
the ping_table struct. There is no current implementation in the
go icmp library to address this problem directly.
To address this issue, I've added a `Tracker` field for `Pinger`
as well as `IcmpData` datastructure to allow for uniquely tracking
icmp requests. The id (as with the `id` field) is not unique,
but will statistically rare for duplicates.
