net: Link Layer Ping

The former ping module was quite useless, but I think the general idea of having a link layer ping module might be helpful for many purposes, not least, for collecting testbed topology information.

I tried to split the PR up in many tiny pieces, following the one-change-per-commit rule, but I'm not sure if this is too much.
I was about to change this, but I realized that it looks bad to. First, I always read `us` (as the objective case of `we`) here, and second, because I still think it's clumsy. How about, I leave the name as it is, but change this to defining the interval in milliseconds, declaring this as the default unit for time constants (as it seems to be already the case for most of the other code)?