ctdb-common: Factor out common ARP code

ctdb-common: Factor out common ARP code

Finding the interface and the MAC address are obvious.  Might as well
set up the common parts of the destination address structure.

Continue to open the socket and find the MAC address first.  This
might seem odd because marshalling and other subsequent steps may
fail.  However, in the future this code might be optimised to open a
single socket to send ARPs for a list of addresses on each interface,
so don't change the logic.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>