add a generic list and make lowpan.c use that list

lowpan.c uses a custom list implementation and intermingles list handling with application logic.
This leads to errors like #299.

I had already implemented a very simple and tested generic list for olsr2, so add that to RIOT and make lowpan.c use it.

It's still missing documentation and RIOT coding style, I'll add that later if you agree with the general approach.

My application runs a lot more stable with it, so I guess this fixes #299