socket-label: move prototype of socket_address_listen() and string table for SocketAddressBindIPv6Only

The function socket_address_listen() is declared at shared/socket-label.c,
however its prototype was in basic/socket-util.h. This moves the
prototype to shared/socket-label.h.

Also, enum SocketAddressBindIPv6Only is not used anymore in basic/*.[ch].
Let's move the definition and its string table to shared/socket-label.[ch].

Follow-up for b25a930f0e.

src/test/test-tables.c

@@ -29,6 +29,7 @@
 #include "service.h"
 #include "show-status.h"
 #include "socket.h"
+#include "socket-label.h"
 #include "socket-util.h"
 #include "swap.h"
 #include "test-tables.h"