Bind source address for cluster communication

This patch allows multiple Redis Cluster instances
to communicate when running on the same interface
of the same host.

The first address given to Redis as a bind parameter
(server.bindaddr[0]) gets used as the source IP
for cluster communication.

If no bind address is specified by the user, the
behavior remains unchanged (if the user doesn't
specify a bind address, server.bindaddr[0] is NULL
and the corresponding bind-to-ip code will be skipped).