Complete net_buffer implementation

[axeld]

There is quite a bunch of functionality missing from the net_buffers, like: - split - have more than one data_node (append/prepend will just fail), ...

[marcusoverhagen]

I implemented split, but in a very dumb way (makes a copy first). I hope this supports further stack development.

[hugosantos]

I think some of these have already been implemented, can you confirm that Axel?

[axeld]

Some, yes :-) However, it's still not complete; just look at how split_buffer() is implemented; it's just a temporary solution.