ieee802154: add rx LQI from userspace

The Link Quality Indication data exposed by drivers could not be accessed from
userspace. Since this data is per-datagram received, it makes sense to make it
available to userspace application through the ancillary data mechanism in
recvmsg rather than through ioctls. This can be activated using the socket
option WPAN_WANTLQI under SOL_IEEE802154 protocol.

This LQI data is available in the ancillary data buffer under the SOL_IEEE802154
level as the type WPAN_LQI. The value is an unsigned byte indicating the link
quality with values ranging 0-255.

Signed-off-by: Romuald Cari <romuald.cari@devialet.com>
Signed-off-by: Clément Peron <clement.peron@devialet.com>
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>