Alternative to #2285. Still WIP (tests need to be adapted).

There were two problems in #2285:

1. To move the pointer pkt->payload_data was not thread-safe.
2. adding new headers would always require allocate new data (just because of the way the pktbuf was handled internally there).

The solution for both is more or less to keep pktbuf a void pointer oriented buffer and (for (1) specifically) to not put pkt_t structs into it. This way every thread is responsible for their own instance of pkt_t and a moving payload_data is not the problem anymore. The header problem get's solved, since the internal organizing structure does not inherit from pkt_header_t anymore now.

Since through this we lost some requirements for pkt, I simplified it a little:

• I moved pkt.h to sys/net/include since it has pktbuf as dependency anyway (technically this is not due to a requirement-loss ;-))
• I removed the packed attribute from pkt_t since it is not stored in the pktbuf anymore
• I removed the member payload_proto from pkt_t since I don't see any use-case for it and we save this way a few byte. It was introduced, since I needed a relationship between pkt_t and pkt_hlist_t to use a modified version of them as pktbuf's internal organizing structure.