sys/net: Add NHDP protocol implementation

This PR introduces the MANET Neighborhood Discovery Protocol specified in [RFC 6130](https://tools.ietf.org/html/rfc6130). The implementation is based on oonf_api and utlist. It introduces a single thread for protocol message sending and receiving.

It is still work in progress in some parts and needs thoroughly testing. TODOs include:
- [ ] Cleanup timed out information base entries
- [ ] Optimize list loops
- [ ] Provide a simple example application
- [ ] Add proper documentation
Would make sense to put this in `sys/include/timex.h`, too.