sixlowpan: rename to sixlowpan_legacy

Since #1680 turns out to become quite a nightmare review-wise, I decided to split and clean it up a little. First point on the agenda is to make it more or less independent from the old 6LoWPAN module. For this I renamed the old module to `sixlowpan_legacy` and I will remove the move commit from #1680. Though I recycled lot of the original code in #1680, it is not what it used to be, so the changes wouldn't be that visible anyways.