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.
We had a very similar discussion here: https://github.com/RIOT-OS/RIOT/pull/1820#issuecomment-60120402 where @N8Fear argued otherwise. I think writing maximum 8 bytes (sic!) twice is bearable for somewhat better readable code.