secchan: Various fixes to run in-band on non-"local" port

secchan: Various fixes to run in-band on non-"local" port

The in-band control code assumed that we would connect to the controller
over the datapath's "local" port.  If the switch had multiple datapaths,
and the controller was only available through one of them, the other
datapaths would fail to connect.  This is addressed by the following
changes:

    - The controller's MAC address is looked up through the connection's
      interface instead of the datapath's local one.

    - We allow ARP traffic to be sent by the connection's interface.

    - We allow ARP traffic to be sent to the controller's MAC address.
      This is necessary if the controller is running locally (e.g. in a VM).

Bug #1466