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/core/socket.h

@@ -6,6 +6,7 @@
 #include "execute.h"
 #include "list.h"
 #include "pidref.h"
+#include "socket-label.h"
 #include "socket-util.h"
 #include "unit.h"