csma: Initialize sequence number with random value

According to IEEE 802.15.4 (§5.1.6.1, §6.4.2), the MAC sequence numbers should
be initialized to random values. This was already the case in
framer-802154.c:create(), but not in csma.c:send_packet(), sometimes causing
false detections of duplicate MAC-layer packets in other devices when a device
was restarted too quickly. This patch decreases the probability of such an
event.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
rdc: duplicate packets: Factor out detection code

The code detecting duplicate packets in the RDC layer had been copied into most
RDC implementations. Factor it out into a new mac-sequence module in order to
have a single instance of this code.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
rdc: duplicate packets: Keep only the last sequence number for each address

According to IEEE 802.15.4e (§6.4.3.9), in order to detect duplicate received
MAC-layer frames, only the most recently received frame's sequence number needs
to be stored for each unique device address. Doing so limits the possible false
duplicate packet detections to a single sequence number. This also allows to
keep the last sequence number of more device addresses for the same value of
MAX_SEQNOS.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>