sys: net: Initial import of a general interface to a network protocol

Interface as discussed in the network meeting last monday. Based on #1492.

TODO:
- [x] define interface
- [x] dummy MAC layer
- [x] transceiver adapter
- [x] adapt for #1492
- [x] provide unittests for dummy_mac
- [x] provide tests for transceiver adapter (?)
- [x] ~~refactor ALL the layers~~ (not as part of this PR)
  - [x] ~~nativenet (partial)~~ (now part of #1492)
Better use `unsigned` for `i`.