net: add IEEE 802.15.4 socket family implementation

Add support for communication over IEEE 802.15.4 networks. This implementation
is neither certified nor complete, but aims to that goal. This commit contains
only the socket interface for communication over IEEE 802.15.4 networks.
One can either send RAW datagrams or use SOCK_DGRAM to encapsulate data
inside normal IEEE 802.15.4 packets.

Configuration interface, drivers and software MAC 802.15.4 implementation will
follow.

Initial implementation was done by Maxim Gorbachyov, Maxim Osipov and Pavel
Smolensky as a research project at Siemens AG. Later the stack was heavily
reworked to better suit the linux networking model, and is now maitained
as an open project partially sponsored by Siemens.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Sergey Lapin <slapin@ossfans.org>
Signed-off-by: David S. Miller <davem@davemloft.net>