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.
`file does not have a C++ compatible header: 'sys/net/include/l2_ping.h'`