sds.c: new function sdsjoin() to join strings.
anet.c: Allow creation of TCP listening sockets bound to N addresses.
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.
Binding multiple IPs done properly with multiple sockets.
Remove cluster stuff from closeListeningSockets() for 2.8.