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)
Variable allocation in 130-133. case-labels are no blocks and with just

```C
{
    /* stuff */
}
```

you get compiler warnings (C++ related, iirc)