Improve `ping()`, create `addResp()`
Improve `ping()`, create `addResp()`

Add `resp{}` as a field of `host{}`. `resp{}` contains metadata (time, seq, etc.) for echo and echo reply exchange.
Made `ping()` a pointer receiver, since we're modifying `h{}` (via `addResp()`).
`ping()` now gets ICMP parameters from its `host{}` receiver.
`c.WriteTo()` and `c.ReadFrom`  under for loops.
Remove log messages from `ping()`.
Add `host{}` method `addResp()`, which adds a `resp{}` to `host.resps[]`.

wgjohnson/chooch