sds.c: new function sdsjoin() to join strings.
anet.c: Allow creation of TCP listening sockets bound to N addresses.
Cluster: use new anet.c listening socket creation API.
Ability to bind multiple addresses.
Revert "anet.c: Allow creation of TCP listening sockets bound to N addresses."

Bind() can't be called multiple times against the same socket, multiple
sockets are required to bind multiple interfaces, silly me.

This reverts commit bd234d62bbfe9feb735fd6d1cdb8f5ce811f54b4.
Revert "Cluster: use new anet.c listening socket creation API."

This reverts commit 016ac38a216d1b701991a451e637ca584f804f93.
Binding multiple IPs done properly with multiple sockets.