sys: net: Initial import of a basic MAC protocol

This is basically an adapter translating `netapi` IPC calls into `netdev` function calls.

Depends on ~~#1448~~ (merged) and ~~#2027~~ (merged)
Yes and no.
1. It's common practice to use this for multi-line macros, where I've rarely seen this in normal code.
2. I think it's not obvious (at least not to me) why one needs a block here.