ieee802154: add netns support

This patch adds netns support for 802.15.4 subsystem. Most parts are
copy&pasted from wireless subsystem, it has the identically userspace
API.

Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>