Introduction of RPL non-storing mode

This is the first commit for discussion. It works - but e.g. the checksum in UDP-packages is wrong because the length is zero (which it is not btw). In addition the design decisions concerning messages sending/relaying may be discuss-able - but I think they are as noninvasive as the network stack would allow them to be.

At last - the proposed fixes concerning storing/non-storing mode files with the same declaration-base in one directory from @Kijewski does not work. (Both - excluding files or moving them to a sub-directory) But I will see more into it. Maybe someone can help.
Ok, I cannot remember this advise.

However, if you envision that the MOP can be switched at runtime later on, this check would be wrong, right?

Moreover, the function is already pretty long (actually, it is too long for my taste), so I guess refactoring this to a separate function would be nice. However, I would ays this is optional, since this is the initial commit, so optimization can be done in a later PR.