net: Initial import of a global packet buffer

Cleaner alternative to #1129. Queue in #1640 

This is basically a static malloc + sugar
`pktbuf_copy()` searches for `packet` first (since it assumes a user oblivious of the internal datastructure calls it) and then uses memcpy(), too, but since we already have `packet` we do not need to search, so it is more efficient to call this internally one. And since we have full disclosure about the memory space here it is also safe to do so.