Answer ARP requests from controller directly in secchan.

Answer ARP requests from controller directly in secchan.

With in-band control, it can happen that the controller needs to send
out an ARP request to find out the MAC address of the switch.  These
packets are sent as broadcasts, so the secure channel will attempt to
send them back to the controller as ofp_packet_in messages.  Of course,
that won't work, because the controller won't be able to respond to
the ofp_packet_in (because it needs the MAC address of the switch to
do so).

This commit makes the secure channel recognize broadcast ARP packets
that originate from the controller and flood them without passing them
back up to the controller.